Just a suggestion...
When I was his age I was terrible at math quizzes like this. If you asked me "what is 4 times 3?" I could tell you "12" right away. But on those weekly page-full-of-problems things...I sucked. While the other kids were racing to see who could finish first I was lucky to get half way.
One day, and I have no idea why, I decided to answer the problems by going down the columns instead of across the rows. We were taught to do the problems like we were reading a book...go left to right.
The day I started doing the problems "up and down" instead of "left and right" I couldn't believe how much faster I went. The teacher was confused at first but saw how many more problems I could do in the time allotted and let me do the quizzes that way from then on. Within a couple of weeks I was finishing them with time to spare.
No idea how all of this works but I guess I just tricked my brain or something. Give it a shot.