I don't like removing him from playgroups, playgrounds
and neither will he,
I feel this is somewhat self defeating he needs to learn to socialise. I do remove him for a time immediately after an incident
unfortunately if it is only for a short time, he will know this, and not be too bothered by the minor interruption,
try to have a mind change, for the foreseeable future no activity will be completed, bonus if they are.
when you arrive at an activity(early), have a chat outside before you go in,
ask him what does he think concerns you about him in the activity, don't tell him, ask him to generate the ideas and concepts, ask him what he thinks he could do to avoid getting frustrated, ask him what he could do to deal with a frustration,
have this as standard conversation before entering places,
"What am I going to say ?"
"What else?"
"What do we want?"
If you supply the ideas, then all he sees is your lip moving and hears bla bla, while he thinks about lego or what ever,
If he has to come out with the concepts then they start to belong to him,
but also explain that if he does get rough, or melts down, you will immediately be leaving the activity, and will have to try another day, and you expect him to say sorry properly to the wronged person.
at the moment you are providing a safety net, as you are becoming very good at predicting his behaviour and watching him closely, so he doesn't have to self monitor,
when you do, do a removal (pre tip off leaders of activities you are on a removal plan, so they don't worry) go home, and be boring, just household tasks, no TV, computers, screens, he has to help or self entertain,
Don't be angry, just say, how disappointing for you, I thought you liked wet play(other activities available), well you can try next time.
You will have to go through this process in order for him to do the joined up thinking that his actions mean he loses out,
Don't worry, as long as you have the pre chat, and are totally consistent with the consequence, he will switch coping tactics very quickly,
always do a debrief afterwards, pointing out all the positives he achieved,ask him what were the good bits and what were bad bits,
He will get there, but the strongest force that will speed the process comes from him, so you have to help him channel it, he has to want to hold it together, at the moment, his want to have control over others actions, is greater than his want to have a hold over his.
try to go as many activities as possible, but with pre chat, and don't see leaving as a failure, it just one step closer to him getting it,
and get him to come up with the ideas so they are "his'.
good luck, you will soon be through this stage.