My Lab/German Shepherd mix is shedding EXCESSIVELY... does anyone have any insight?

He is 18 months old, and does not have a thick coat. His coat is thinner than a lab's, . He actually has places on his underbelly that are completely bare (it has been this way since he is was a puppy). We got him bathed this past weekend (we bathe him every 4-6 weeks), and since then he has been losing an excessive amount of fur. If I run my hand through his fur, I end up with a FISTFULL of hair. I can see all the roots... it's pretty disturbing. He does have natrually dry skin and hair, and has been on a nutritional supplement since he was a pup to help with that. He eats Science Diet dry and wet food. He is a very well taken care of dog... gets daily walks/runs, goes to the vet regularly. I called the vet and they said I did not HAVE to bring him in unless I started to see bald spots, but I'm thinking about taking him in anyways. Does this sound familiar to anyone? I've never seen a dog shed like this before. Could it be the dryness of my home or may it be serious?