No it's not. A 17 year old isn't the age of majority and cannot be contractually obligated. Why do you think he had to get his mother to co-sign? The car dealer isn't stupid.
If the car gets repoed and someone starts calling him, as soon as they find out he's not 18 they are going to forget him and go after his mother. The worst that will happen to him is a black mark inserted into his credit history, which frankly isn't going to make a damn bit of difference nowadays since creditors don't seem to give a rat's ass about good credit anymore, they will finance anyone now. Do you know that credit card companies will issue credit cards to people who have just declared bankruptcy? It's rediculous.
Ted