First, let me say that DD3 lost a few words around that age, and I got all panicky about ASD. Now I feel silly for even thinking it--she's now absolutely symptomless. Of course that doesn't mean that you can rule it out just yet.
I'm not an expert, but I think that reading words he cannot say is a sign of hyperlexia. I also think that it is possible for a kid to be both gifted and hyperlexic. I can't tell from your post whether he has any of the other signs, but you might look into it.
Good luck. Your son sounds delightful.
