We have a cat that puls out his hair, and has no fleas. It is caused from allergies, and when it gets bad he has to get a shot from the vet. When my one indoor/outdoor cat needs flea meds, I use Advantage, and have good luck with it.
was he showing active flea's or did you just use b/c you thought you needed too? these are pesticides and are harmful if used too frequently...I would give him a good bath and see if that helps, pulling hair out can also be a sign of allergies.
heres a link to frontline info
http://www.simplesteps.org/greenpaws-products#/h/+/+/623