My youngest DD7 sounds much like your DS. She is one of the youngest in her class, and is currently not grade accelerated but is in a 2nd/3rd combination as a second grader but doing 3rd grade reading and math. Either her FISQ or GAI (I can't remember which) was 139 and she loves math.
If I had your options, I would definitely do the 3rd grade math. If you are worried about gaps, I would just keep handing him math workbooks and let him go to town as long as he wants to do them for fun. Good luck with your decision!
