I would just follow his lead and keep it fun. Play games - add grapes at the table, find patterns in the flowers at the park, skip count money, cut apples into fractions, if each of your 3 friends gets 2 balls how many balls do you need? etc. I guess You could get him number magnets to play with though. Look at what he's learned already on his own! I wouldn't formally introduce math at this age. I also have a math gifted kid, and this is the kind of stuff I did with him as a preschooler and he was well ahead in math by kindergarten.
