My assistant has high social anxiety and can be very hot and cold

My first thought was he may have Asperger's. I have worked with a number of young people who display very similar behaviour to this and it can make life difficult for both them and those around them. I may be wrong but is there any way you could find out? Maybe ask him when he seems calm. Somehow, once you know the situation you can usually find ways to handle it. It sounds like he's not having an easy time of things either. Good luck.

He doesnt sound like a great assistant to be honest

I also immediately thought aspergers

My first thought is you need to manage him out...
balling up fists in rage at a standard request is not normal or acceptable office behaviour.
He also doesn't sound like he is particularly good at assisting you which is his job!!!

I have social anxiety OP, and I work in a busy office mingling with people all day long. The fist clenching is not acceptable regardless of any disability in my opinion. I am the yes yes people pleaser you described, I will say yes to everything to avoid confrontation ( workload willing ) and I go above & beyond to be polite, approachable & part of the team regardless of my sometimes daily struggles with my condition. He sounds very difficult to manage, and I'm not sure it's social anxiety, more like he can't deal with the type of work due to whatever disability he has. How the bloody hell did he get the job? Ones character would surely be apparent in interview you would think?

I’d have a chat with HR, say you’re concerned he’s not coping, which I don’t think he is. Then have a chat with him, ask him for feedback on how things are going and tell him you’re concerned and what you’d like to see improving. Are you changing your approach to work tasks based on his behaviour?

I sounds like he doesn’t like being told what to do. Have a chat with him for his own sake, or he’s going to make himself unemployable.

My first thought also ran to "Autistic spectrum".

I've worked with a lot of Autistic adults in my time, and he sounds fairly typical - but of course, we shouldn't be diagnosing via the internet.

The fist balling is totally socially unacceptable, but honestly, he may not even be aware he's doing. A lot of people with Autism do that as a stimming behaviour (self soothing), as a feeling of bodily pressure or tension can actually reduce anxiety.
You say that he does it in response to a change? Yeah, it's likely a way for him to manage a sudden onslaught of sensory stimulation/thoughts.

He sounds hard work, but a lot of his behaviours are explained if you look at them through an Autism understanding.

Do you have a HR department? I'd go to them for advice.

If he has diagnosed specific needs of some kind, then the necessary adjustments should have been put in place long since. A friend’s husband was diagnosed as on the autistic spectrum in his late 40s, and adjustments have been put in place in his (university admin) workplace.

As matters stand, what you have is someone behaving unprofessionally. How do you know he has ‘social anxiety’?

If I had asked an assistant to do something and they sat with their fists balled, frowning, I would ask them if there was a problem as they seemed annoyed about something. I would address it directly, not ignore it. But it sounds very stressful for you.

He clearly hasn’t been diagnosed with anything if you weren’t told. Are you his manager? If not then go directly to his boss and explain everything with specific examples and try and arrange a formal feedback that can be used in the disciplinary process further down the line if he doesn’t improve. If you are his manager then follow your disciplinary process as he’s not performing - mention it during your 1-2-1 reviews, put him on a performance improvement plan if required, then revisit this regularly.

I worked with a female like this, I would bet my house she has aspergers.
Agree re the treading on eggshells - it was not pleasant. But when she was fine she was great. The issue was never knowing which version you would get

I think you need to develop a very hard skin and not take any of it personally.

thick skin not hard!!

At the moment you are not dealing with a person with Autistic traits, Aspergers, Social Anxiety or anything else. No one has made you aware of any problems along these lines, therefore as far as you are concerned you are dealing with an awkward NT male who possibly resents your higher position and being instructed by a woman as young as you. That is where you have to approach the problem from unless a proper diagnosis comes to light.

Go to your manager/HR and say you have concerns he is not coping with his job very well and explain his reactions. Do not make any assumptions of your own based on his behaviour or you will be the one in the wrong.

Good luck and remember, until you are officially informed otherwise he is an NT difficult person.

Aspergers? My son has this. It should be noted if he has this and work will have guidelines on reasonable adjustments that will be made for him. Speak to hr and see what they can do for him if he isnt coping in that role.

aspergers might be likely, but in the absence of any diagnosis, theres no way you should have to feel intimidated or aggressed in your workplace

I see I've been beaten to it, aspergers was my first thought too. You could be describing my husband. He doesn't even realise he sighs and huffs, has no idea what that his tone can be off and he sounds pissed off.
It can be difficult to navigate.

OP,
Reading about your assistants behaviour makes me realise that coming to work every day is not easy for him.
He is trying very hard to fit in to society's expectations but it is a massive massive struggle.

His behavior is not aimed at you - these are his internal struggles

I do not agree with the comments above about 'managing him out' as that is cruel IMO and not very humane

How about getting to know the guy, and supporting each other.

I mean one job loss could be the beginning of a long unemployment / life spiral, for this guy

Don't be the person who does this to another human (who is struggling) just to make your own life easier.

Try to be a better person, and we lift up those who are struggling - not put our boot on their head

Sounds exactly like my kids dad. He has aspergers and the huffing and puffing is very familiar.

I suggest you sit down with him and explain the behaviours you've noticed and ask about them ('Ive noticed sometimes after I ask you to do something, or we've had a chat, you sit with clinched fists at your desk looking stern. I'm wondering if this is because I've annoyed you? How would you like me to ask you to complete your tasks in a way that is more comfortable for you? And doesn't lead you to feel angry or frustrated?)

As someone upthread said, you've been given an assistant who is behaving in a way that undermines his and your ability to do your jobs properly. An assistant who is often inflexible, irritable, sometimes oppositional and whose mood swings, behaviour and responses are difficult to predict and deal with. It's not your job to diagnose whatever developmental disorder, MH problem or learning disability he's struggling with.

I would get into a pattern of asking what's happening when he displays difficult behaviour – when he's irritable with you, when he's displaying anger, when he's unable to talk to you. Make notes of particular incidents (days, times, what was said, time lost as a result). Gather what information you can – he may or may not be able to explain what he's experiencing and why – and then take this to your manager or HR department. I might say I was finding him very difficult to work with (cite incidents) and that he was struggling too. Whoever appointed him can't, surely (from what you describe about his behaviour) be unaware that he has communication and behavioural difficulties?

80daysaroundtheworld, that's a noble sentiment – but really, why should the OP have to spend so much time and effort building a relationship, tolerating very difficult behaviour, handling him with kid gloves, phrasing requests in ways that suit him, never asking for last-minute changes and so on when she's there to do a job and so is he?

The support, the therapy, the training needs to come from higher up. She's a very young woman: why is so much more expected of her than of him? I do think there's a strong assumption that women are innate psychologists/ therapists/ mothers and will use their special womanly skills to sort everyone out and make everyone feel good. This really isn't a good thing for women. We should just be able to get on with our work without having to look after others.

Hopefully he's got a probationary period and you can get rid once it ends?

Social Anxiety is a horrible horrible thing to live with, I know because I live with it and I hate going to work and having to comply with social norms and fit in with people who feel the need to hide their insecurities by putting others down. Not being aloud to be different because other people aren't, he probably just resents being there, and doesn't care what you do because the best case scenario is that he gets sacked and doesn't have to tolerate another day there as he can't leave or the job centre will sanction him.