Leaving is always an option.
I have several ocd, agoraphobia, depression and anxiety myself, but I try my very best to be as well as possible.
It's particularly unfair if a person with poor mh expects to be supported by others but doesn't do anything to resolve this themselves.
I am working damn hard to get better by engaging with therapy and taking meds I'm not totally happy with I'm about to start some new ones on top of what I already take. I was basically abandoned by local mh services for 2 years at one point when very ill and fought to get the help I needed.
"Being there for him" doesn't mean enabling him, being co-dependent or allowing him to treat you badly there's no excuse for that.
I've had moments when I'm in a panic attack and I can lash out (verbally) but I accept that is unacceptable behaviour and instantly apologise to whoever has been on receiving end, I'm mortified when it happens! But to avoid it happening I know I have to do the work to avoid panic attacks.
People who moan without doing what's possible for them to either change the situation or change their ability to handle it are not acting in their own best interests and are not being considerate of those supporting them.
Perhaps that approach MAY work with him, point out that getting treatment for possible depression not only will make his life and health better it benefits the whole family (and likely from sounds of it his colleagues)
He can't go around like a bear with a sore head if he can't be arsed to take the aspirin!
My ex was just a lazy, entitled dick! Certainly at least work wise. He's the youngest of 4 and while my ex in laws are lovely and generally good parents they did spoil him in certain ways (his siblings certainly thought so) and baby him. He joined the army at 17 and I met him around 2 years later and he had a huge chip on his shoulder assuming promotions should come at a certain length of service and when that didn't happen for him he didn't understand/wouldn't admit it was cos his bosses NOTICED He was a lazy sod! With few leadership qualities. My dad is also army, most of that side of my family are military, so I grew up within it too. My dad was professionally very successful one of the youngest rsm's in his regt at the time, BUT he went above and beyond - without being a mug - and was very skilled at his job. I remember quite early on in dating ex his surprise at my dad volunteering to do an evening class related to his "trade" even though it wasn't mandatory. My dad always wanted to learn, to improve. Ex did the basics, did THEM shoddily and then expected high praise! If anyone asked him a favour he'd be "what can you do for me?" Or only do favours for those he considered potentially "useful" to him.
I remember one argument in particular when a friend of his with 2 years less service than him got a promotion he thought he was owed. He came home raging (which I wasn't having!) and I pointed out to him that the friend had worked to gain related qualifications at night classes and was therefore more qualified on paper than ex AND he was happy to extra curricular roles (voluntary roles within army life that frankly gets you "brownie points" for promotions as the military is a community and the bosses want to see that you understand and contribute to that) AND he grafted at work too, he didn't mind working an extra 20 mins, doing an extra duty here and there - again without being a mug about it. That's why he got the promotion.
Ex declared him a brown nose! And the friendship ended.
He declared ME unsupportive for giving him some home truths!
When wife 2 and I were still talking regularly I learned he was just the same with her.
His temper became more of an issue and he was actually disciplined out (ex and wife 2 didn't tell me this heard "on grapevine military esp army is a small world and I'm still friendly with some from then) - which rarely happens these days! He was apparently repeatedly questioning his boss, refusing to do tasks assigned, or delegating them to others when he'd been specifically told he was to do whatever it was, being "off" with subordinates etc - it wasn't a one off incident it was a pattern of behaviour
I see your partner heading that way if he doesn't rein himself in. And I'm sure you don't need reminding that if you're sacked from a job it makes it harder to get another and he could be prevented from claiming benefits for a period of time too.
He needs to grow up - part of being mature is recognising that your unwillingness to eg get a medical condition dx and treated can impact negatively on others and that's no way to behave, it's also recognising that you don't get nothing for nothing in this world! You want a good career with a good salary you have to earn it. Not just by doing the work but by doing it graciously and understanding you have to pay your dues!
Several of the wealthiest and most successful people in the world started off cleaning, shelf stacking, fixing photocopiers, selling tat at market stalls etc and worked their way up.
I left school at 16 with 3 GCSEs, got a full time nmw level job (except no nmw then I was on £1.50 an hour) and did Eve classes to get more GCSEs and then a-levels, worked full time for a few more years until I could "qualify" as independent financially from parents (as they couldn't support me for other reasons) so I could go to uni.
I hold 2 degrees and have some damn good experience/refs but I've worked all kinds of jobs from shelf stacking and bar work to elderly care management and all sorts in between.
You do what you have to.
I'm disabled and unable to work now I'd love to be able to work again I refuse to accept I never will again despite some around saying I may need to.
He has a job a lot would love to have I'm sure, but he doesn't appreciate it and at the moment he doesn't appreciate you properly
As I said support him if you want to - but don't enable him to continue with this attitude he has on him at the moment.