I am not a professional but what you are describing sounds exactly like a gifted child on the spectrum to me - huge vocabulary but no pragmatic language or interest in communicating with others. Avid interests - but entirely self centered/driven, with zero interest or tolerance for other peoples ideas, interests or activities.
I understand that you feel he's happy in his own world and only unhappy when in therapy - but to me that's kind of the point. While he definitely needs time to just be, and to be happy, he does also need to be learning the skills to function later...
His therapist doesn't sound that skilled so a full evaluation sounds like a good plan.
the problem is that some of those could be either Autism OR Apraxia / dispraxia OR both. I've been doing some reading on Apraxia / Dispraxia and how it differentiates from Autism and one thing that stands out for me is that with Autism the child is stuck on rituals versus with Dispraxia the ritualistic behavior is not there. He's never had any behaviors like that (unlike DS4.5 who has PDD-NOS diagnosis) and while he wants to do his own thing, he has no problems with transition. I've always said he's our "easy" baby. He's just very relaxed and goes with the flow. He IS very self directed.
Even his Developmental therapist and my older son's OT who've known him for about a year don't quite believe he's Autistic. They say gifted and very stubborn and with sensory processing issues (that run in the family). I would think that since he just turned 3 it's too early to evaluate his pragmatic language skills? But I'm not sure. It's one of the reasons why I'm getting an independent evaluation. ... we had DS4.5 speech re-evaluated couple months ago because I was worried about the pragmatic part of it and the evaluator said in the end that DS just has too much to say and makes it look like his pragmatic language needs help.
