I would let him plow ahead. I don't believe in limiting a fast learner to try to make him like the slower kids-- I think it can do much more harm than good.

Where we went, or continued to go, at the end of second grade math:

Singapore Math 3A and up, plus Challenging Word Problems / Intensive Practice
Mathematics Enhancement Programme
Sunshine Math
Ed Zaccaro's Primary Grade Challenge Math
Raymond Smullyan
Building Thinking Skills
IXL (rarely used now if ever, but excellent for fun practice; can be subscribed to on the cheap through the Homeschool Buyers Co-op)
ALEKS (just signed up-- there's a sale on until the end of 2011)

... plus assorted games.

Striving to increase my rate of flow, and fight forum gloopiness. sick