He does mental math requiring keeping track of numbers (i.e. 42 times 37).

I notice that being able to enjoy juggling large amounts of digits requires a certain predisposition.

I do lots of math with my 9 year old and he likes to do (and discover) tricky (at his level) properties of numbers like noticing that 3 cancels in 3/2 times 4/3 (he doesn't know the 'rules' governing multiplication of fractions but somehow juggles the numbers nonetheless - in this case he notices that 3/2 is 1,5 is one and a half...). I'm quite sure he enjoys it as I talked about this exercise with his older brother when he chimed in. But he doesn't like mentally multiplying two-digit numbers (though he can). He rather does rough estimates.

Why am I telling this? There are differnt kinds of working memory and other brain functions. And doing multiplication exercises can be fun but need not. Why do it? Could be that it's easy/natural. Could be that it is fun. Could be that it is challenge. Could be that it is hard work but wanting to conform to expectations may do the trick. What's it in your (sons) case?