I would pre-arrange a ride if you know anyone else coming back to your area from the party. Not sure if you have a budget for it but stay at a cheap roadside motel just for the night would be something to consider.
One question I have to ask is, are you really going to have a great time though seeing him drink all night if you don't want him drinking around you or your son? Won't you all be in the same location? Are you going to avoid him all night? That would be sorta stressful if I were trying to have a good time. Just something to think about.