Sounds like he wasn't allowed to grow up because he was the youngest. Probably had a constant narrative of 'oh, you aren't any good at that, I'll do it'.
If you asked my DNA sharers, I'm an utter incompetent who should probably not be allowed to dress or feed herself without paramedics on standby and every attempt to do something as a child/teenager was met with 'you can't do that, you'll just break it/burn the house down/damage yourself/we know what you're like'.
DP, who is admittedly shit with anything that involves spatial awareness (so he wouldn't drive) or anything more complex than a single, linear approach to a task, grew up in a kinder version - but it still came down to 'you're crap, best not even try'.
The difference between the two of us is that I'm a bloodyminded old besom who knew very early on I couldn't trust the abusive morons in my 'home' to tell me the sky was blue, so didn't accept their opinion of my abilities, whilst DP was loved and encouraged to never try because he'd only fail and they wanted him to not feel bad about himself - so he didn't have the automatic distrust of anything he'd been told about himself that I did.
Two children in the same house can equally grow up in a completely different family with different roles given to them - your DP could have been The Eldest, the Helper, the Sensible One, the One Who Copes whilst his DB was The Baby, the Hard Work, the Silly One, the Clumsy One or some other such label.
I'm not suggesting that you keep him in that incompetent and helpless role, but I'm also not suggesting you refuse to help him at all; what he needs is firm but kind 'I'll show you how to do it if you need to, that way you can do it yourself from now on'. And then if he refuses, it's fine to say 'I've shown you how to do it, now it's your turn, it'll be fine' because you aren't just leaving him to sink or swim.