D.B.
I guess you could do genetic testing, but I will bet that nothing will show up. If it makes you feel better, then at least it's something you can ask for.
The hard part in finding a diagnosis that is elusive is that he's so young. What I really think you might need to do is get him as much early intervention as possible. Keep working with him, provide consistent boundaries and discipline, have all his "team" on board with the overall plan, and wait this out. When he is around 7, you can do more testing, along the lines of CAPS, dyslexia, etc. Have an OT work with him as well as the speech therapist. And get the speech therapist to do some real work with him on receptive language skills. The more she understands this part of him, the better.
If he is not able to handle a regular classroom later, you might consider a Montessori approach. The more you can do to help him NOW, though, the more chance that a private school will accept him. Otherwise, it's either an IEP in public school or homeschooling.
Good luck!
Dawn