ulcerative sores on face

Greetings,

My 23 year old son spent time at a nearby park five evenings ago with his younger siblings, mostly playing in the stream. The next morning he had two somewhat small sores on one cheek, the skin gone in those spots as though he had scraped off a layer of skin while shaving. Those then scabbed over, and over the next several days more such areas have developed and scabbed over as well, with the original ones still there, (I think). We assumed that he had come into contact with some plant and was having an allergic reaction. Given his schedule I have not seen the growing problems until this evening. He has washed with fels naptha soap and that is the extent of anything we have done. It hurts, especially when the areas are stretched, but his pain threshold is fairly high and he only told me that when I asked. It does not itch. Now he asks me what he can do. Please direct me in this. I don't want to make it worse for him so I am hesitant to pull out my "arsenal" of alternatives without some strong counsel. It will probably resolve itself on its own, but if I can offer any relief I would be happy to do so.

Thank you so much,