Your first step should definitely be the vet visit. At 3.5 years of age, it isn't normal for a dog to have joint problems. You'll want to rule out dysplasias and diseases that can cause joint pain--are you in an area where there are ticks? Many of the tick-borne diseases can cause joint pain and are readily treatable with antibiotics.
If he does have joint problems, getting his weight down will help a lot. Glucosamine/chondriotin supplements may also help.
As for the flea baths--you can get topical treatments for fleas from your vet that will work much better than OTC flea meds/shampoos and not dry your dog's skin!