Great to hear Sampsom is doing well
Sucks about the fleas. It's been a long time since I've had to deal w/fleas, how is he with daily combing? Will he let you sit him on your lap to run through parting the fur checking him over entirely for fleas? I used to do that when my last ones were kittens with a cup of soapy water nearby to drown any fleas I caught.
Be sure to treat the house environment as well as the cats - frequent vaccuming, hot water washes of bedding etc.
Absolutely check with your vet first, also be sure that you are not using expired product. One option is to apply treatment to the other cats but not directly to Sampson.
Advantage and Revolution cautions against using their flea product on sick pets.
Frontline Plus for cats doesn't have a warning against using on sick cats aside from their sprays should not be used on animals that are geriatric or sick.
The other major treatment is Capstar (makers of Program) which is in pill form, it looks safe with low indications of adverse reactions but to be on the safe side I would caution against this as it is cleared by the kidneys.
I think in this circumstance my
would be Frontline Plus for cats but applied to the others not Sampson. I have not used Frontline before but have used Advantage for cats on healthy cats and Program for Dogs.