Not sure how old your son is, but this is what I'm doing with my 5 year old with the same issue. Find some good old-school funk from the late '70s / early '80s with a four-on-the-floor type of drum beat. Zapp, Cameo, Gap Band, or something like that. Get your son to count along "1-2-3-4" with each of the beats and clap his hands on "2" and "4". Once he's reliable at doing that when you don't start it for him, then get him to stomp his foot on "1" and "3", or just "1". Once he's got that down, there are all sorts of things you can do. He can copy rhythms you clap, or if he's good with notation, he can practice writing down a rhythm you clap.

Alternatively, you could always get him drum lessons. This is the meat and potatoes of what a good drum teacher would cover.