I probably shouldn't be admitting to this ... but pretty much all he knows is from his video games or tv. Anytime I had the need to "teach him" something, I found out he already knows it. So I gave up and figured there's plenty of time for real learning when school comes. But it does make me wonder sometimes how much more (or not?) would these two kids know if I actually did try to teach them things above and beyond of what they pick up on their own?
My son learned so much from video games and tv that I didn't limit his time on them. We unschooled from first grade until 8th grade except for math. The games he played always seemed to have history or literary references. He looked up a lot of things on Wikipedia because of video games. When he watches tv, my son analyzes the episode just as he would a book. We recently started reading Paradise Lost and as I was reading he asked me to look the book up on tvtropes.com because he recognized some familiar tropes and could name a lot of manga characters and shows that were listed as having used these tropes. I think he sounds almost like a college professor when he analyzes tv shows and games and because he can sometimes talk like a college professor there have been people who suggested that he might have Asperger's but all of his doctors said he does not have this.