our cat does this too, but shes still a kitten, so im hoping she will outgrow it.
obviously, he hasnt differenciated between play and hurting. there has to be a consequence, yet you cant haul off and hit the cat. so what i would recommend is spend a weekend waiting for this to happen (hopefully to YOU and not your child) and squirt the cat with water at the moment that it happens. it shuoldnt take long for him to figure out that when he he does that, he gets sprayed. usually sprayers make a noise, but you can further accentuate that by hissing at him the way a cat would hiss you know?? this helps because USUALLY the moms will hiss at their babies when they are being bad. if your cat was separated from mom too early, he might not have learned that at all.
also, one more thought; maybe hes bored? you could consider getting another cat, or you could just make sure that you are playing with him more often during the day, or giving him opportunities to play, hunt and chase. there are cat toys that will hang from your door or something, my cats loved to "stalk and jump" at that toy ... until it broke LOL.
anyway good luck