1. He shouldn't call you names or fight over it
2. He shouldn't work on your car, but his
3. He'll never get better at any of the things he does because it sounds like there's an expectation he should get it right the first time like a professional
4. You have to dabble to find that thing you do best and enjoy doing

So while you don't want him messing with your car instead of his (he's trying to do a "guy thing" for you which becomes emasculating because he can't do it) he needs to find his way and have ups and down so he can get good at something he likes. Sounds like you both need to back off each other to me.