A natural skin tonic can be made by thinly slicing a whole lemon with the peel. Add the lemon to 1 pint of near boiling water and then let it steep all night. The next day sponge it onto the dog and let dry. It has d-limonene in it, which is a natural flea repellent. You can use a commercial flea soap with d-limonene in it.
Or add a few drops of pine or eucalyptus essential oils to a gentle liquid soap to make a natural anti-flea soap.
What type of food is your dog eating? If there is any way to switch the dog to a completely raw food diet, this will ultimately be the very best thing you could do. Commercial foods compromise the immune systems of pets, which results in allergies, among so many other maladies.