Every soft toy or material he has been in contact with needs to be washed and dried. The whole house needs to be vacuum top to bottom paying particular attention to the baseboards and carpets. Make sure the vacuum bag is put in the trash outside.
Can a ferret get the flea drops that dog and cats get on their back? Maybe ask the vet if he can have a flea collar. Give him another wash after the whole house is done and perhaps he needs cream rinse so his skin is not dry.