What about using a computer program? There are online programs such as ALEKS that are fairly inexpensive that use AI to determine when a subject is mastered - hence eliminating useless repetition.
My son just used regular software like Cluefinders 3rd grade to learn and move further in math. Although - it covers more than just math. We often had little short lessons in math and he would push as far as that understanding took him in the game, then he would ask another question and we would repeat the process.
Not too many books written for young kids with the intention of young kids reading them and teaching themselves math. Maybe a math workbook - with permission to only do the pages he wants to do?
