I am not sure about the sores, but my son had a case of bad cradle cap minus the sores and this was how we got rid of it.
Use a beanbclean brush while washing hair.
http://www.amazon.com/Bean-B-Clean-Scalp-Massaging-Brush-...
I was given a very similar brush when we left the hospital, but it broke easily. This one is much sturdier.
Massage any kind of oil on the scalp. Baby, olive, coconut etc.
Take a baby comb and back comb the hair very carefully, a little at a time. The cradle cap will come off in large flakes (unfortunately, the hair will come off too). We did this from 3.5 months to 4 months and it was gone!
I would not comb until the sores are under control. My pediatrician had given us the go ahead to comb his head and pull the flakes off. He wasn't bothered by it at all.