"You get what you pay for." you're doing this for free, you shouldn't have to worry about it. he is choosing to let you do it. Like the rest of the quarter of a billion of Americans - he has a choice who does the work. He's not doing the work, he's not willing/able to learn it, he's getting free labor, and a free car (you insinuated you "picked it up")? The best value in this whole car deal is the present opportunity to teach some values, economics, skills, and opportunity costs and more...
I help people all the time with cars - my rule has always been "i only work on cars for free". I will not accept payment. And I tell people - I might screw up. I rarely do, but sometimes i break something or something is collateral damage for certain jobs. They pay for it, no big deal. No one ever minds - they're unbelievably thankful I'm saving them hundreds (or thousands) of dollars.
You should not have to think about possibly breaking something for your efforts and lack of his.