I’d say hives. It could be from bug bites or sweating. Or a reaction to something once he started sweating. If it returns, you can use some rubbing alcohol in water and sponge over the areas to help a bit.
They were all over his body (not face), mostly on his hind. It’s minus degrees celsius where I live, the indoor is heated though. The bumps were soft and not sore. I’m thinking something to do with sweating but I’m wondering if there’s an actual name for it