Whether or not the contract is enforceable against the ex-girlfriend, it is enforceable against your son.
He is not necessarily liable for the balance of the lease. The landlord would have an obligation to find a replacement tenant using reasonable efforts (this is called mitigation of damages). If you son is able to find a suitable replacement, that would also help and reduce the amount he would have to pay.
Forcing the girlfriend out might be a tricky matter. If the contract with respect to her is found to be valid (e.g she is emancipated or turns 18 and ratifies contract, or contract found to be a necessity, etc.), then he has no right to force her out. If it is not valid, he might run into other problems, such as the fact that she could possibly be considered a minor under NY Law during at least a portion of the relationship Those are tricky waters and I would urge you to advise your son to tread carefully there..
Lawyer
Extensive experience and training in real estate matters.