At 13 my HFA son made these decisions for himself. If he went to a party, it was because he wanted to help a friend celebrate, and not because he expected to have a great time. Yes, by 13 he was capable of thinking that way. If he thought the party activities would be too uninteresting for him, he knew he had the choice to decline (and he had no trouble doing so).
I don't think you need to worry about whether or not he feels entertained. The job of a party is to be fun for the birthday child. And while you can hope it also appeals to the guests, they aren't really the point of the party. And at 13, kids usually know that.
Not to mention, kids often LIKE doing things from their earlier childhoods. Fun memories
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).