That was my first thought also. Does he sleep without a shirt? I've seen a lot of memory foam mattresses that have a similar pattern or cover. If it's always when he wakes up, I would be looking very close at the surroundings, especially since there is no pain or swelling, raised bumps, etc..

I haven't had it happen much, but no doctor has had an answer for me, I'm currently waiting for another event so I can hopefully make it to my doctor's office, and take photos ... My wife has some I believe... But any time we noticed them was after waking up and always on my back.