Maybe you could do some really hard math at home, and give him a script to use when frustrated, and practice the script. (This can be an "asking for help" script, or a "coping statement" like "I think I'll try this problem again another way" or "If it's wrong, I'll have a chance to fix it later.")

This sort of skill (the coping with frustration) needs a lot of practice before it's second nature.

DeeDee