try to find a picture of fleas on the internet then you will know what your dealing with.
if you cant see anything in his fur then it might actually be something else like mites. i use advocate on my cats because it gets rid of all types of parasites including, fleas, worms, mite, ring worm, lung worm etc instead of just the fleas that frontline do. id would recomend the advocate instead, you can get it from yur local vets and they sometimes do deals too. Best of luck, Amy