There are no books on baking formulas.
Formulas are used in commercial production, so textbooks that cover the topic will not be geared towards the home baker. And they do not actually explain Baker’s percentages, Rather they explain formulas, how to read a formula sheet, how basic formulas are developed.
If your son has difficulty with math he will probably have difficulty understanding a textbook explanation of formulas.
This is a page out of one of my textbook. No where does it ever actually explain how to calculate baker’s percentages. Rather, it explains how to read the formula sheet. The assumption is if you understand how to read it, you’ll understand how to calculate the percentages.