Mt daughters male cat has become COVERED in scabs! What can this be?

They are little round ones; around the base of his tail @ the ocks, along the spine, sparsely around his belly, and badly on his head and around the base of his ears. I treat him with Frontline when I can afford it, but he is an indoor cat & RARELY goes outdoors (ONLY when he sneaks out!) & I seldom see him scratching. It seems to have gotten slightly better now that the weather has turned colder however, it will not completely go away. What could this be and what treatment options should I pursue?