L.M.
How old is your son? I would suggest just jumping into it. You will be surprised how quickly he might catch on. If you've done addition with manipulatives, then show him a simple problem, like 1 + 1. Say something like, "If I have one bear (point to the "1") and then I add another bear (point to the other "1") how many bears do I have now?" If he gets it, great! If he doesn't, try putting the "bears" or manipulatives over the numbers. That way he'll see that the numbers actually represent the manipulatives. After a while, make sure he understands what a "plus sign" is and what it means. Also introduce the "equal sign" to him. I am a teacher and I have come across some great websites. Type in "Kindergarten math lessons" in google and see what pops up. I hope this helps.