Sounds like he may have a yeast infection. I'd try lotrimin (generic is fine)on the rash, then put LOTS of diaper ointment over top. With a large of amount of diarrhea, the rash will likely only get worse as it continues. He probably does have a virus and will continue with these symptoms until it's over.
Meanwhile, make sure he keeps hydrated. Diarrhea can easily dehydrate a child. Also, when changing his diaper. Forego the diaper wipes. They contain soap, which will sting very badly. Instead, use warm water and a wash cloth (gently). I know that can get messy, but water will be the best option for a bad rash. You may even want to put him in a bath or small tub of warm water to wash him off(without any soap).
Hopefully, after a day or two the severe rash will start going away. If you don't see some improvement after a day or two. I'd call his Dr. They may be able to prescribe an ointment for him.