It might be higher than you think. Unfortunately, IQ is only a proxy for intelligence. With “normal” people, it’s quite a reasonable proxy. At the extremes it begins to break down.

It might be worth trying him on some of the easier IQ tests. Autistic minds tend to like focused puzzles, which make up the bulk of IQ tests. That’s actually the source of the “idiot savant” stereotype. A massive over-focus on a small subset of “puzzles”