This looks like a form of contact dermatitis. It would seem from the regular pattern that your son's skin was in contact with a fabric or substance - either surface or underneath - that is causing a reaction. Especially in our hot sweaty conditions this summer, it could be a car seat, life jacket, lawn chair, the inside of a foam pillow...
Or did he recently get new sports clothes? Sometimes the sizing or fiber can be ab issue. Start a new sport? The disinfectant they spray on shoulder pads and the like is nasty to tender skin.
If is becomes itchy or weeps, or keeps increasing, a visit to the dermatologist (even a video appointment) could provide the answer.
Let all of us inquiring minds know if you can an answer please.
Sue