As well as the other good suggestions above, there is EPGY (now called giftedandtalented.com) which has an online K-7 math course. The "Independent Study" version costs $60/quarter (=$20/month). You could play it safe and start at grade 3 if you fear gaps, (and there's some set theory and logic in there you don't see in school courses) and he could probably work through to end of grade 7 in less than a year. Make sure it's on "gifted setting". It's a bit dry but he can learn the material quickly.
If you choose the EPGY route, I’d follow 22B’s advice about the gifted curriculum setting. I’d contact giftedandtalented.com before you sign up to see if it’s still available, and I recommend you ask whether the Initial Math Placement Test that was available in the EPGY version is still available as well. There were two versions of the Initial Math Placement Test: a 45 minutes or 48 exercises whichever comes first version and a 100 question version. The 45 minute version is sufficient and very accurate for placement.
