As I'm sure you realize, arthritis refers to swelling of the joints. A rash on the skin on top of the joints does not imply or suggest arthritis. Although I'm not sure exactly what this rash is, I doubt it has any systemic significance. (I assume your son is otherwise healthy.) My advice is to set up an appointment with a skin doctor. If possible, take photos of the rash when it's in full bloom, in case it isn't there when you see the doctor. If the rash disappears before you get the appointment--and that wouldn't surprise me at all--I'd forget about it. Lots of funny rashes come and go without meaning anything.
Take care.
Dr. Rockoff
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