It's a surprise.
He probably doesn't even remember that tomorrow is his birthday.
But we're ready.
There will be cake, and a build-your-own-burrito bar, and music. No Kool-Aid. Lots of friends, and one very embarrassed birthday boy.
He'll grumble about putting on pants, no doubt. He'll likely express amazement that people would want to come to a party for him. He'll eat too much cake, and too many burritos. He'll want to play "just one more tune". He'll crack yet another joke about being the "oldest Daddy in town".
He'll have a wonderful time.
Happy birthday, dear Himself.