I would get him tickets. Invite him, but do so AFTER he's had his visitation right before the event so he has no chance to say somethng stupid to your son. Let him know that if he's going to say something rude, he need not come...but a real dad shows loving support to their child. He should too.
If you get him tickets, that means he's NOT backstage with your son. He has to watch the show. Also, don't sit with him in the stands so he doesn't have the opportunity to mouth off.
Afterwards, I'd make sure that there are plenty of family/loved ones around to support him. Have someplace to go after (whisk your boy off to have ice cream to celebrate or something). With lots of other people around, hopefully your ex won't say anything.
You need to put your foot down on this behavior once and for all....start with making it hard for him (the ex) to even be in a position to say anything.
Best of luck!