How about 1yr grade skip with subject acceleration in those areas which matter most to him? Then if that's not enough, consider another skip next year so it's not so large a jump?

Is DS unhappy? I don't think they need to be challenged in every area unless DS is craving that.

I'm sure many kids do fine w/ a 2yr grade skip. I just watched a show where they have a transition year for 13yr olds who then attend college full-time after completing that year as 14yrs. What a jump that is!