Dealing with a negative family member…or a person who just likes to put you down

Hearing that other family members are also noticing his behavior is a big step towards either helping you correct it, or justifying your next move of correcting it yourself. As for the car? He has no input into what you do with it anyway, so my advise is to enjoy building it the way you want, and sharing it with like minded people such as here, your parents, and any family members that you get along with. He will eventually be on the outside of everybody he pisses off, and you will be none the worse for wear.