*puts groomer hat on*
The first thing you'll need to do is treat the underlying cause. Baths are great, but if your pet has food/environmental allergies or fleas, all of the baths in the world won't fix it. If your pet's skin is broken out in bumps and irritated areas, there is a good chance he has some sort of superficial infection caused by the scratching, which needs to be seen by a vet.
That being said, medicated baths are a HUGE help with skin/coat care with dogs that have compromised coat health, when paired with a treatment plan for the underlying cause.
A good thing for red, irritated, majorly itchy skin is going to be a combination shampoo of ketoconazole and chlorhexidene. Virbac makes a product called Ketochlor, I do believe. It may be a prescription only shampoo, not sure, but it does its job.
Virbac also makes a product called Allergroom, which is a hypoallergenic shampoo ment for dogs with allergies. I love it, and it is my go to shampoo for dry, itchy skin. Cynder is bathed in it in the summer months to keep her coat healthy due to her mild allergies.
Oatmeal shampoos are good for dogs that are sensitive to shampoos themselves, but honestly, if the dog is already itching and scratching, they don't seem to do much of anything, IMO.
*takes off groomer hat*
Hope that helps!