We did the birthday party! We had previously "bought out" of the previous two years of birthday parties. It went well!
Everyone who RSVP'd actually showed up, and some kids from camp we had him invite manually did as well. We were worried about this; though the guy has gotten better at making friends and has made several friends at camp, he didn't make any at school last year, and his not really paying attention to other kids was flagged as an area of concern, hence the social classes.
The kids had fun, and even did the treasure hunt activity. A small girl and some park rando had actually collected all of the coins I had hidden before I announced the start of the hunt. Fortunately, I had not tried too hard to hide them and was able to quickly re-hide them, though not without being noticed by the kids halfway through, so it turned into a chase kind of thing.
A friend from camp that we had heard about showed up, and he was this super outgoing dude that went around introducing himself to all of the parents. His mom spoke only Chinese but was really personable. Pretty brave of her, I thought, though she did have her other 16-year-old son as backup. He was very different from his little brother; a quiet dude in a Keith Haring shirt that went to boarding school.
A lot of the kids developed this almost cinematic coin craze. We explained that they were working as a team to collect coins to get prizes, but each kid was wary of letting other kids count their coins so we could get a total. In the end, I just said I would have to take them and count them. (It was kind of a fake count anyway, as I was going to say they won all of the prizes no matter what the total was.)