You've got to get the fleas in your area under control. Bi-weekly baths in medicated shampoo aren't good for his skin (as you have seen). Are the fleas in your house or just in your yard? I would spray, fog, fumigate, get a contract with a pest-control company, or do whatever you've got to do to get the fleas out of the environment. If it's so bad that your dog has to get baths every other week, chances are the fleas are on you and your kids (if you have them) as well.