Two adjacent spots on the left side of his neck - can you see them, just below the bottom end of his freezebrand, and above the wet hair?
This one's a little harder to see, but: look at the corner formed by his wet hair, just up from the bottom of his neck. Now track up. It looks almost like a coffee ring on his neck.
On the left side of his neck, it's almost like the hair is thinner and darker simultaneously. On the right side, it's just discolored.
One theory is that they're just weird bug bites. He has always reacted excessively to bug bites, from huge seeping welts to hives.
Another theory, and I am leaning toward this one, is that these are reactions to his IM Pentosan shots. But three separate spots would mean that the oldest of them is at least 3 months old. I don't remember seeing them even two months ago, much less three. It's possible I'm just wildly unobservant about things that are not his legs, but I don't think so.