If the gritts flea dirt comb some off him onto a peice of damp kitchen towl & fold it over , the gritt will have a red halo round it if its flea dirt
Its basically dried blood
i wouldnt bath or treat him with anything chemical , untill he is stronger , the dirt will work its way off him with a gentle comb & if you change the bedding every day if you can he will soon look like a nice clean ferret