Does he prefer to have his face sleeping on the side? DS2 was quite a sweaty baby and always slept with his head one way and developed quite a heat rash (very similar to this) on that side.
It seemed to come up more when he was feeding? Maybe because he was working up a sweat! I didn't use anything but a cool face washer, it seemed to go away after 12ish weeks.
I'd still raise it with the GP.