Does your son have Asperger's Syndrome? This level of intense focus on a narrow interest that interferes with ordinary social functioning is one of the diagnostic features.
Speaking as the mother of an AS, homeschooled, 14 yr. old, strong-willed, hg/pg son who has had an intense interest in tabletop wargaming since he was very small, I sympathize. One strategy you might use is to encourage him to express his interest in ways that incidentally included other things you want him to learn without explicitly pointing this out to him.
For example, when my son was about 7 or 8, we introduced him to the BBC History website, which includes simulations of historical battles, and in his explorations of those aspects, he eventually began reading other areas of the site and this broadened his interests.
We encouraged him to make maps and write narratives of his campaigns and share them with us and with others, bought him a cheap old version of Age of Empires (which included a great deal of history instruction in the background text included in the game),encouraged him to learn about the military in different countries and historical periods, discussed the different social and cultural factors required to support armies, the political, social,cultural, religious and economic factors underlying how and why wars start and end, the logistics involved in warfare, how warfare has driven innovation in industry and medicine, inquired about how crossing different terrain would impact the soldiers (Would it be colder or warmer in the mountains? Easier or harder to breathe? Would there be water available? Food? Fuel?)to stimulate learning about geography and science, we went on field trips to site of historical battles, and did everything we could to spark an interest in him learning things that would expand his horizons by connecting them to things he was already interested in.
Some of the kinds of question we would ask or express an interest in were similar to these: How much water would you have to carry with you if you were marching an army from ______ to ______ ? How could you figure that out? Would ground troops or fortifications be better to secure a border from attack by foot soldiers? By artillery? By cavalry? What if the border was a river running through a plain? A river at the bottom of a valley? The top of a mountain range? How can you figure that out? Can you convince me of your position using evidence from science and history? What made the Samurai such good warriors? Was it the same thing that made the Scots good warriors? What about the Romans? The Mongols under Ghengis Khan? Why are some cultures more war-like than others? How do geography, religion, natural resources, and economics influence that? What materials have been used for armor historically and in modern times? Why? You get the idea...
All of that said, we have a rule that he needs to work on something school-related for at least two hours every single day. If he can make a convincing argument that what he is doing counts as school, if he can point to the learning that is happening, and how it is relevant to developing the skills that he needs, then he gets to count it as a school activity. For a while, when his keyboarding skills were still at the point where he needed to improve them, he got to count playing Runescape as typing practice up to three hours a week. This rule about "If you can convince me..." has greatly improved his skills in argumentation and rhetoric, incidentally. We also have a rule which says that my final answer is final, though, to prevent incessant arguing.
It gets better, if you can avoid turning it into a power struggle which you both will lose - which can be the most difficult part of the whole thing. My son, at 14, is active online editing a gaming-oriented Wiki and writing not one but two blogs related to his gaming interest, both of which have numerous followers. He has developed online social relationships with other bloggers and wargamers, and he socializes IRL with others who share his interest at our local gaming store, which hosts free open game days and nights open to players of all ages. The strong interest is still there, and probably always will be. But his interests and knowledge base are pretty wide ranging at this point, although there have certainly been times when I wanted to despair. He is active in Aikido, he goes swimming with a few friends, he participates in a homeschool book club, he writes awesome poetry (some of which is haiku inspired by his wargaming, but still - he is writing poetry!), he is working through the Hippocampus AP classes in math, biology, and US History this semester, he is using Academic Earth and Annenberg media courses on his own as interest strikes, he lists "Arts and Ideas" among favorite books, and he participated in a Duke TIP law class this past summer and excelled in it.
There is hope. I wouldn't blame homeschooling. You might need to change how you are implementing it though, and even then, it doesn't sound like you got an easy child. Mine has always had "a strong will and a strong won't", and has been "high maintenance" since infancy. I'm glad that you posted this, and I hope that what I have written is helpful.
Edit: In case I didn't make it clear, I support the idea that you should get your son evaluated by a psychologist.
