What are the brown spots?

our son has these too. He has some behind his ears. it almost looks like dirt. He gets it in his ear too. I try and try to wipe it away but it doesn't come off. then tonight I noticed he had it under both armpits as well. he does have skin issues but hasn't had any in awhile other then very dry skin. he doesn't seem bothered by it. I tried putting lotion on it tonight. i think we will need a trip to the derm. again. he hasn't been to one since he was 1. he will be 4 in June.