first off take the flea collar off him asap, it could make him extremely sick or even kill him. Only use for them is in a vacuum cleaner bag. I hope you went to the vet for flea drops, OTC meds expecially Hartz flea products have the same effect that flea collars do, negative!
If you used an OTC flea medication, bathe your cat in Dawn Dish Soap. You want to get that stuff off as best as possible. The Dawn will kill/stun fleas to get them off the cat and down the drain.
Then get a flea comb and comb over the cat really well. Dip the comb in soapy water each time you comb over the cat.
See how that goes and then treat him with a flea medication from the vet only. Like Advantage or Frontline. I would deworm him too because fleas and worms go hand in hand, if a cat has fleas you can just assume he has worms too.