I think going the unschool route sounds great. Maybe sit down with him and find out what he wants to work on? If writing is an issue for him, maybe getting him something like Handwriting without Tears and telling him if he can do one page a day, that would be good. I've found just by doing that one page a day, my kids have gotten much better and much more comfortable with writing and it takes like 5 minutes. Manipulatives and computer games work well with the math.
I have 2 homeschooling kids and as long as they're at least at grade level I often let things slide. My DS9 has decided to unschool science for the time being. He clicked with writing out multi-step math problems earlier this year, so my evil plan with stalling to get to "real" algebra has paid off. I let him do one math exercise a day, but then he does programming or other math games. I don't have a GT kid that clamors to get through a math book (although he had one 2 week period where he got through about 1/2 year because he was enjoying it). My 5 year old is often done with her "work" in 15 minutes. Often it involves oral math or games. We do a lot of hands on field trips and activities.
Hang in there! To me it sounds like you're doing ok, but I totally understand that feeling of wanting something "concrete" that they're working on. I think whether they're in school or not, they ebb and flow.
