Hi-
I have 2 children, both with Sensory Processing Disorder. Even from very young ages, they weren't very social, preferring to play by themselves with THINGS rather than PEOPLE. They have both been evaluated and had OT,and seem like totally normal kids now, who enjoy peer friendships.
You have a couple of options, if you are concerned that his behaviors are sensory related. I would recommend reading Sensational Kids, by Lucy Jane Miller, or The Sensory Sensitive Child by Smith and Gouze. The "bibles" of course, are The Out of Sync Child and The Out of Sync Child Has Fun, both by Carol Stock Kranowitz, but personally, I found those less reader friendly than the other two. There's also Raising a Sensory Smart Child by Biel and Peske.
Meanwhile, call a local pediatric therapy center that has one or more Occupational Therapists trained in sensory issues on staff, and arrange for an evaluation. I tell you to do this, because you may have a bit of a wait. OTs are highly in demand. If, after reading the books, you change your mind, you can always cancel the evaluation.
Second, you could contact your local public school district and voice your concerns about his social skills and difficulty with transitions. They are required to set up a screening for your son free of charge. A note of caution about school districts, however. Some believe in sensory processing disorder, and some do not. It varies from district to district, and the diagnosis is not yet officially in the DSM-V.
My son's sensory issues were severe, and qualified him for Early Intervention, and eventually, district preschool; however, the concerns over sensory issues by the school district seem to "disappear" suddenly when he moved to kindergarten. Many of his issues were suddenly labeled "home issues."
Anyway, I wish you luck. It sounds like you may be on the right track. Pediatricians don't always know about SPD, either, so don't rely on your son's doctor. I belong to a couple of online support groups as well, so as you are making your journey, please let me know if I can help you with other resources or information. BTW, many children with sensory issues are also gifted, but not all. :)
Feel free to e-mail me off-list.
M.