Where this kid's parents even there? I would have called his parents to come pick him up. If they were there, they weren't dealing with it. They probably should have taken him home, rather than leave him like that at the table. You could have suggested it, "Maybe you need to take him home." You can try to engage him in conversation (ask what his favorite subject is at school or if he's looking forward to spring break), and see if that makes him feel more comfortable, but if he completely shut down like that, just let him be. There isn't much you can do at this point besides try to ignore the discomfort. If his parents weren't dealing with it, I don't know what more you could do. I think his parents should have taken him home.
I'm wondering the appeal of the tween to attend a 4-year-old party at a bouncy house. It sounds like he had fun and made the best of it, but he was really not connecting with anyone. He wasn't part of the adults and he wasn't part of the kids, and he was ready to go home. He spilled his drink, was embarrassed, and was pretty much topped off at this point. As far as what you can do? I don't know. Maybe when you plan something like this again, plan something that has a greater appeal to the older kids and that they have something to do to entertain themselves. You can limit your guest list to children your child's age and his friends, and have an adult get-together another time. You seemed to try to engage this young man and appease him, and beyond that, just leave him alone. Call his parents to come pick him up, but if his parents were there, and they were leaving him alone, leave him alone too. As parents, there are many times we have to leave early, even when we don't want to, and I think this is one of those situations that they should have left early and taken him home.
And I agree with:
Two options, depending on the boy (and I realize you may not know him well enough to decide which one is the "right" one).
1) Ignore - lest said, soonest mended.
2) Share a story of when you did something equally mortifying.