Hi there. The fact that you have still seen the odd flea despite the flea treatments means that this is more than likely a flea allergy- that means that he has an allergy to the flea saliva(they inject some when they bite).
The best thing to use to treat fleas is a product called 'Frontline' which you can buy in pet stores as well as on-line and at some chemists. It kills the fleas before they bite which is vital.
The other thing you can try, to stop the allergic reaction, is antihistamine tablets, as they can help stop the allergic response that leads to itching, you can try either Benadryl(diphenhydramine) at a dose of 1mg/lb every 8 hrs, or Zyrtec(cetirizine) at a dose of 0.5mg/lb every 24hrs.
The vital thing is that he has no fleas at all on him, and you may also need to spray your house with a household spray, as they live in carpets/soft furnishings, and not on the dog.
Andrea
Dog Veterinarian
bvm&s degree, 15 years in companion animal practise