Sounds like a really fun time....Everyone getting drunk to celebrate, or in remembrance at a funeral, or because it's a holiday, or because it's Saturday. Hmmmm, that sound like me!
Alcoholics are the evolution of the perfect ego manic. It's all about them(me). When loved ones buy in to the manipulation - hey, it's my birthday weekend and my nephews - we enable.
Boundaries are critical to break that insane cycle. If he wants to drink himself to death, there's nothing you can do about it. But, WHY as a mom would you continue to expose your child to this behavior. Unacceptable IMO.
Be strong, establish visitation boundaries - ie, NO drinking by dad around my child EVER - and stick to it.
All that said, it's easy to type advice out for someone else via the internet. Only you know the truth of the situation and circumstances.
Peace to you