Hi S.,
It sounds like an allergic reaction to something. Most reaction will start on the torso. My daughter was allergic to strawberries, she has since outgrown it. But when it first happened she got the red bumps all over her belly and it moved around to her back. So I had to retrace my steps and figure out what I had fed her. It finally dawned on me that I was giving her pb&j, but normally I gave her grape and this time I was giving her strawberry. So long story short, lol, I think it is an allergic reaction to something. It may not even be food allergies, did you change laundry detergent, fabric softner, etc? Has he been in a pool alot this summer or in the ocean....it could be exposure to those elements as well. I hope this helps. J. D