Can I combine Seresto with any other flea product?

Another possibility is that he's having an allergic reaction to the bites from the fleas. Flea bite allergy is extremely common, and just a bite or two can cause itching that can last for weeks. You might try giving him an antihistamine to see if it helps. You can find dosing info online for giving dogs Benadryl, Claritin, and several other OTC antihistamines. Or your vet's office would likely give you the correct dosing info over the phone.