?

I have a 9 yr. old Russian Blue mix that in the last couple of weeks has developed little red scabby bumps on the back of his neck and his shoulders that seem to be getting progressively worse. By progressively worse, I mean that he keeps getting more little red bumps...the individual sores are not actually getting larger, just multiplying. His disposition is normal and because I have 2 other cats that are NOT exhibiting the same symptoms, I assume it's not fleas or anything else contagious...plus they are indoor. Any insight would be greatly appreciated!!! Thank you all!