In real life he's the second worst driver in the world behind my daughter, so I'd definitely make him a very slow tractor.
I think the reason he's no good at driving is because his sister has had so many accidents that he's just nervous. The majority of her accidents were the other driver's fault, but I told her she needed to take a defensive driving course because I think she could have avoided some of them. She's in college now, though, and doesn't particularly listen to my suggestions. If I paid for her college she might, but she's on full academic scholarship. She even pays for her own apartment. I do pay for her damn car insurance though, which is like sending two kids through Harvard.