My 23 month old is just getting over a bout with diarreah that started at the beginning of December. His stool is still loose, but it's not the terrible stuff it was for so many weeks. I had him in to the doctor as well, but there's nothing they can do about a virus. Make sure he gets enough to drink... push water - it's the best to keep him from getting dehydrated. That's the thing to pay attention to. One way to make sure he's got enough to drink is if he's peeing enough. Check with your doctor about frequency of wet diapers.
The other thing is to make sure your diaper cream has 40% zinc oxide. Desitin original has it. My little guy's bum was so angry red and he was filling his diapers with such toxic waste that I couldn't get it better.
Make sure to use lotion all over his body, too. My little guy gets dry skin and he was scratching all over... had to keep his fingernails clipped regularly! After I started using lotion (right after bath works for us) he stopped scratching.
Oh, I almost forgot! We also put him on lactose free milk for a while. The Dr. said that when they get a stomach bug like this, it sometimes takes a while to get the enzymes that break down milk back up to a level that will work. He's back on regular milk now and doing fine.