Well, here's the truth, sorry.
It's in a public place and unless YOU have a protective order against him there is no legal way you can keep him from being at the same public place you are. They can't make him leave, you can't make him leave, you can't call the police on him, unless he hits one of you while in the place he can come and go as he wants to.
So.....find a way to deal with this happening. Tell him he's going to have to plan his own party on his own weekend and invite his own family....
BUT expect him to show up. You and YOUR family have to be adults and accept this. Your son does NOT want to remember that one birthday party when he was a kid where it was ruined because mom wouldn't just let it go and make nice with dad.
Not saying this is your fault, only saying he's going to show up and your attitude during this party is going to make the party or break the party.
If you're nice and just stay out of his way, surround yourself with your friends and family and just ignore him when possible, then chances are he'll be on his best behavior too.
I'd say expect him to come, be the bigger person and stay out of his reach, then let your SON have the best party ever.
Next time when you are fully divorced dad showing up is a no. He shouldn't even know anything about anything that goes on when kiddo is not at his dad's visitation.