D.C.
If this has only gone on a couple of days (or less than a week), I wouldn't worry. His digestive system is probably out of whack. For me and my kids, after a stomach virus or such with diarrhea, it takes a while for us to want to eat much again.
Try some bland foods to help him get back -- the BRAT diet is always good -- Bananas, Rice, Applesauce and Toast (dry). Small bits of Gatorade also help (we all think Pedialyte takes yucky). Milk will make an upset stomach worse. It's okay for him to go a few days without it.
The best thing I've ever done is to think about how I feel when I have diarrhea or a stomach virus and what makes me feel better for the next several days and then do that for the child.