yes 'been together' for 15 years but vvv casually hence not living together.
I am totally bewildered by this - how on earth can you can be with someone for 15 years, vvv casually?
Especially when they have a whole house to themselves, bought and paid for - why on earth did you never move in?
This is a very odd relationship indeed. I understand your part in it no better than I understand his, to be honest.
But he is not your SON, he is your boyfriend. And has been for 15 years, where you have not progressed your relationship, where he has not managed to keep up his house but let it fall in to disrepair, where he has made several attempts at both education and work, but not gotten on with any of it….
At the same time he suffers delusions of grandeur, feels above work, and now expects you to start paying his keep?
Hello? Does it not bother you?
Completely agree with whoever said this. And I am not really sure what purpose would be served by seeking a diagnosis at this stage, except to give him more ammunition to justify doing nothing useful with his life, or for bailing on everything he starts that he then deems to be boring or beneath him. Being forced to knuckle down to something mundane but solid might actually be the best thing for him. He sounds like he's avoiding cracking on with real life out of a mix of fear and a misplaced sense of superiority.
He's clearly capable of succeeding in higher education, he is able bodied, his intellect is not impaired, so even if he never has the career of his dreams due to being a bit flaky or too difficult to work with, he should be perfectly capable of holding down a low to medium grade admin job in the public sector where it's harder to be sacked if you are rubbish or perhaps in academia, which would surely suit him. Or just any old run of the mill minimum wage job just to get him functioning and contributing like a normal person, instead of sitting up in his ivory tower wringing his hands and waffling on about how no-one understands him. 
Whether he has Asperger's or not, if he can manage to find and keep a relationship for 15 years and he can manage to see through all these educational commitments when it suits him, I'm pretty sure he can do whatever it takes to hold down a basic, undemanding job of work. He just needs to focus on what he's doing that gets him laid off each time, and consciously work on doing it differently next time. Another bloody degree is the very last thing he needs.