Hi @looloo84 That’s the craziest thing, isn’t it? I’ve not experienced this myself but find it so fascinating that there are people all over the world experiencing this with no rhyme or reason. There doesn’t seem to be any correlating exposers, ages, genders…
I mentioned to 2 new members who recently joined with the grid rash, @redgrids, @sheryldenise, that hopefully with more members who experiencing the rash write in and a commonality can be exposed!

Does your son have a fever at this time?

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