
My Dobe has had countless skin problems, and also had terrible acne on his chin as a pup. The skin specialist he sees recommended Panoxyl Acnegel 5. It contains benzoyl peroxide and is designed for acne in people. I got it from Boots (OTC I think) but you can get it online
here. It does bleach fabrics though so best put on before a walk. You can also get it in 10% strength but she recommended we start with the 5%. I found it worked brilliantly - the description says results within 2 weeks but I was noticing a difference within just a couple of days.
ETA: it's also advisable to change any plastic food/water bowls to stainless steel.