Can you get some appropriate level workbooks from Singapore (or similar) and get them to agree to let him work on that during math time?
It's kind of a lousy substitute for actual instruction but it may be a more interesting activity to do practice work at his level. Or instead of practice work you could look for some appropriate problem solving work. If the teacher doesn't have time to work with him, he could work on the problems himself during class and you could go over it with him at home.
My DD did EPGY 3-4 the summer before third grade. It goes up to introducing long division. She really liked the geometry stuff in EPGY. They hardly do any geometry in school.
Good luck...
Cathy
