My oldest daughter had that when she was about 9 months old. It freaked me out cause I didnt know what it was I thought she had the measles. So i took her to the er. It was in fact Roseola. Just keep him cool and in less cloths as possible. lots of fluids from the fever. It should go away in a few days to a week.
There are lots of diseases that present as a fever (sometimes quite high) that breaks followed by a rash. Sometimes there is weird poop. Sometimes not. As stated: Lots of fluids. Simple foods. These illnesses are highly contagious but by the time the rash shows up the contagious level has dropped to almost nil. These incubate for a week to 10 days before symptoms show so you can never tell who is sick until...it's too late and all the kiddies have it.