Dog's loss of hair and irritated skin

My boyfriend's chow-rottweiler-lab mix (about ten years old) has been having some problems with his skin. Lately its to the point that the top of his rump, his tail, and the insides of his legs have lost most of their hair (though I partially attribute this to chewing.) His skin has patchy pink on it, and some scabs. I have tried to give him a bath every week to clean it up (I use oatmeal shampoo) but when I thought it had cleared up I slacked off, and it turns into this brown buildup on his skin, like skin oils mixed with dandruff and dirt. It was manageable over the summer (we thought it was just allergies) but has actually gotten worse since winter came.

Is this allergies to something inside, or a skin infection, or something else? What would be the best way to manage it, or treat it?

He eats Pedigree adult dog food, and two big tablespoons of pumpkin puree (for the fiber), and has been taking Brewer's yeast tablets since he was small.

[Misfits4Me]

Welcome....From your description of the areas effected, this is classic of Flea Allergy Dermatitis......

[Misfits4Me]

Something else:  Upgrading his food to a higher quality would help all around, also.......Karla

[Margot49]

Agree with Jarla. That's what it sounds like.  Are you seeing fleas? Get a fine toothed comb and put it slowly against the lay of the coat so you can see Diwn to the skin, look for fleas and flea dirt.  If you suspect fleas, you need to do flea treatment inside and outside and treat the dog. If that is the case, I suggest you contact your Vet and get the dog on a monthly oral medication.
Agree also to get him on a better food.  Try to find a grain free food with Omega 3 in it.  Do a gradual transition over a few days mixing old and new food.