I started him on a first grade math workbook from the bookstore and he blew threw that like it was nothing. I grabbed a second grade book and so far that is easy for him as well. I picked up a third grade book because I don't expect the second grade book to last long.
I can heartily recommend Singapore Maths Primary series (available from Amazon) as it builds a very solid foundation. There are also SG Challenging Problems books too which really work those concepts in deeply too (think mental kneading as in bread making).
