OP, this sounds like my sis and BIL.
He is ASD, has some degree of dyspraxia, is dyslexic, had a very strange upbringing, and has suffered some loss of cognitive function due to seizures brought on by him mismanaging his diabetes.
He was incapable of sorting out pretty much anything. It was, for my sis, like having a giant child to look after, and given that she didn't want children anyway, hugely exhausting and disappointing.
A few years ago she started doing what she calls "BIL Independence Training" where she gives him tasks to sort out on his own - and rewards him on a sticker chart.
I realise this sounds totally patronising for a grown man, however he has actually revelled in it and in the last couple of years especially has started being so much more of an adult, he's almost unrecognisable.
This was a man who didn't know how to register with a doctors or how to pay a council tax bill or umpteen other everyday tasks - he is now taking part in a hobby which means he goes away about once every two months, deals with all the travelling on his own, books his own tickets, finds his own way there (he is appalling with maps, probably because of the dyslexia) and has been able to make future arrangements about things (a massive deal since he never had any grasp of planning before.)
Do you still love your DP, or has it all trickled away in frustration? If there's any hope, he will have to accept that his lack of ability to do anything is seriously impacting on you and your DC, and that he has to address it, as scary and difficult as it may seem.
If he won't accept this, and is really living in cloud cuckoo land, then for the sake of your mental health and of your DC, you may have to separate.