You cannot control what your parents do. Perhaps you can talk to them ahead of time and explain why you think it is best if they do not help him (which may or may not work, but it's worth a shot, right?). They are enabling him if they "help" him, and they need to understand that. Other than that, you cannot control them anymore than you can control your son. But if anyone is going to enable him, do you really want it to be you?