In MA there is a lemon law, even between private parties. I had to take advantage of it because I bought a Chevy caprice in the want ads whose catalytic converter was hollowed out so the car wouldn’t pass inspection. I was 17 and didn’t know to look for that.
So while I think your situation dictates you are in the clear to explain that you don’t owe him anything, I might also brush up on your states law first.