Hello and welcome, I am Dr. B and would like to help you today.
Unfortunately Frontline hasn't been working as well against fleas as it used to. Therefore, you will need to consider a newer alternative for your cat. Ideally we'd want to use a more updated product like Advantage II or Activyl. Otherwise, if you are being bit, you may need to treat your home since fleas will invest the home too. When doing so, do make sure to use a product that also contains insect growth regulator (IGR) to make sure you kill off the larvae, eggs, and adult fleas. Finally if he is scratching all the time, you can need to start your wee one on an OTC antihistamine ( ie Benadryl - 0.25 mg per pound every 12 hours) or have his vet use a long acting steroid injection to at least settle the allergic itchiness from the fleas saliva.
Please take care,
If he has only had it a week ago, we'd need to treat the house this week and plan to use the new treatment next week since we do need a two week gap between the two.