We use an electronic timer. Started with adding single digit numbers. He needs to provide answers before the timer goes off. Right answer adds a dime, but wrong answers or no answer before time expires subtracts a dime. Identified the numbers he had trouble with. We play until he’s taken a couple dollars from me. I always let him win a couple dollars to keep up the interest. Lowered the time until it was down to a second.
Most math is learning and applying a technique. But there is no technique or formula for adding/multiplying single digit numbers - it’s all memory. That’s what I did with my grandkids, and it frees them to learn the techniques without struggling with the basics.