I wouldn't try a flea collar or drops these could make him very sick. They need to ask the vet if a ferret can tolerate a low dose of Comfortis. I live in FL where the fleas Never die. They can be dormant for months until a host happens by. Comfortis is one of a new class of flea killing agents that a dog takes internally. I think there is a cat version also. Must be taken on a full stomach and we break the tablet in 1/2 and give the doses 3 days apart. It's expensive so we want to make sure it doesn't get thrown up. It is a miracle drug, fleas are gone within the hour- it's amazing.