Where he's got a bit of a bald patch on his chest, it looks like rain rot (I'm assuming that's what rain scald is called in the U.S. As that's the only name I've known it by), but the rest of his shoulder looks like hives, like he's had a reaction to something. Some type of plant maybe managed to weasel its way into the rug when he laid down or rolled, and worked its way back out as he moved around. If it were food related, he'd have hives all over, not just on one spot of his body. If you can, I'd get some MTG, and put that on there, and leave it on, reapplying it every day that you go out, and see if that doesn't help, but give him a really good grooming first. Good luck, hope it clears up soon.