Smow, Advocate didn't suit my guys (and yes, fleas are getting immune to Frontline). We swear by Stronghold now, it deals with some of the worms as well.
One of my old cats had fleas, they didn't appear until she left - they lie in the carpet waiting for warm blood to walk by, then they hatch and jump.
Make sure you hoover well under the front edge of all the chairs and sofas - that's where ours were lurking.
Don't dose the cats again, it's much too soon, and the fleas aren't on the cats - they've dropped off and laid eggs, it's the new fleas which are biting your son.