I have no advice for you. I did have a friend who did something similar and eventually that obsession was replaced with something else. Have you spoken w/ his pediatrician about this? What about older kids that can play the math games with him? HOw does he do then? My only naive advice would be to get him out and about as much as possible to zoos, museums, etc to broaden his world beyond his obsession w/ numbers.
Not to feed his obsession, but to breathe new life into Chutes and Ladders we play it with a pair of dice. You can add or subtract the dice to put yourself in a more favorable position. You can even get negative numbers so you can move backwards if it'll get you to a ladder or helps to avoid a chute.
