I know that it is quite a frustrating process to identify a diagnosis. Of course, a diagnosis can only be assigned based on a thorough, in-person assessment by a qualified professional. In most cases, a diagnosis is helpful primarily because it provides a means of obtaining services for your child. Most diagnoses don't really explain behavior nor do they specify a treatment; they merely provide a label for a group of characteristics. Your primary concern seems to be your son's problem behavior (e.g., noncompliance, aggression). A large body of research suggests that behavioral interventions are effective in the majority of cases, independent of the diagnosis. I recommend that you identify a Board Certified Behavior Analyst who can help develop a program to improve his behavior. A list of professionals is available at the website of the Behavior Analysis Certification Board (bacb.com). If you don't intervene soon, it will be much harder to control his behavior as he grows.
I wouldn't rule out Asperger's because he is social and can play in his room for 2 hours by himself. My daugther, who has Asperger's Syndrome, can play all day by herself, and can be social, too. However, how is the social interaction? Is your child the one "in charge" of what they play and how they play it? I think you need to find out what is causing the behavior before you can try to change it. Strategies that work for neutotypicals does not work with Aspies. A behavior analyst would be a great start.
Your son and my son have a few things in common. My son is 14 and was only just diagnosed with Sensory Integration Disorder/Dysfunction. I would strongly suggest another evaluation with a different Neuropsychologist and Behavioral Health Consultant, They test in a variety of modalities and can give you a better understanding of how your "gifted" child can be affected by any one of a number of things (like SID) that fall under the GIANT umbrella of disorders/dysfunctions that are in the family of Autism Spectrum. Now, don't get wrapped up in the lable "Autism Spectrum". It tends to freak parents out. Keep your eyes and ears open to the explanation of all possibilities and treatments given to you by your next professional evaluator. By the way, my son has always been in the "gifted" catagory, read by himself at age 4, can build anything no matter how difficult it is for an adult, is a whiz at math, skipped a grade in school, BUT HAS ALWAYS HELD HIS EARS, even when "anticipating a sound". he could play alone for many hours by himself when younger, and has at the same time been extremely social and well liked by all adults. sometimes other kids found his precocious manner of speaking "funny", and if you ask him what time it is he will "tell you how a watch is made" in great detail. What I am trying to say is, when a person is affected by something that falls under a "spectrum" disorder, you can't expect it to be black and white, or one thing or another. Its frustration for all, especially when it seems the child is being argumentative or defiantly disobedient. As for the sensory integration, there's much to know about the vestibular system, tactile sensitivity, etc. Again, you don;t have to have "all of the symptoms" to have the disorder. But a thorough eval and diagnosis can set you on the right path. We have had treatment with an OT and continue to do the "Therapeutic Listening Program" at home. I can honestly say it has helped. The times my son will try to put his hands over his ears are far and few between. His OT has also helped him to have better balance, go on theme park rides he could not previously tolerate (roller coasters, etc.) and his core strength is better. The pencil holding/handwriting is something that we will also need to work on. But the help is out there. You just have to find the right people. Keep trying until you find it. Oh, and by the way, my son is in the 9th grade and earns excellent grades in honors classes. We have to stay on him to make sure he "follows through" with assignments and "pays attention" and "listens", but sometimes you have to do that with kids who don't have these types of disorders. Just follow your hunch, don't get sidetracked by the fact that he is "smart", and help him (now) so that when he is on his own he can function in a very impatient, loud and busy world.
Best of luck.