DS is expressing interest in learning new skills, so that why Khan seems good. However, what I watched seemed to move kind of slowly. He is in a weird place with skills, I think. He has a lot of mental math abilities from playing games for older children (can add large numbers accurately in his head using...I guess you'd call it decomposition skills..."1300 plus 850...I know 1300 plus 700 is 2000, so then another 100 is 2100, then the 50 is 2150" ). He could not do that problem on paper in the traditional way, though. And there are definitely holes in what he knows. I don't think he really knows much about fractions, for instance, and he can't tell analog time past the hour and half hour (no analog clocks in our house--oops). Similarly, he randomly knows a lot of multiplication stuff but hasn't been systematically taught anything.
Your son sounds like mine - does quite a bit in his head and LOVES to be quizzed on math that the older kids are doing ("ask me a times table!" "Ask me a square root!" He thinks it's fun!). One of the reasons we started Xtra Math was that he was clearly learning times tables by osmosis or something (DD had learned them), and I was frightened that he might be doing it without mastery of addition and subtraction facts. Xtra Math is actually a bit of a challenge because you need to answer in less than 3 seconds and it is a challenge to even type that fast! So this might keep him busy...for a little while.
I think you will need to experiment a bit to find out what he likes - I think at this age you want him to enjoy whatever he does, so if he won't mind the drier format, go for it! I agree that you should try open enrollment and try to get it set on "gifted" settings if you use it, though. I haven't tried it with DS yet, because he still feels like math is just FUN! When I hear DD doing EPGY - it doesn't sound like much fun (sorry, EPGY!). Truthfully, EPGY's lecture-style just will not appeal to every bright younger child, but some younger children would probably be fine with it. You'll have to experiment and see what your DS likes - I think flexibility with these kiddos is key, because they learn and change so quickly!
