Did your son perhaps eat hard boiled dyed easter eggs or help dye them?? If so, he might have salmonella poisoning and probably ought to go to the doctor for antibiotics. It could also simply be one of the nasty stomach viruses which can last from 24 hours to 10 days!
If it is one of the stomach viruses, all of them are highly contagious, so your son needs to stay home from school until his stool is basically solid and everyone around him needs to make sure they wash their hands a lot. I actually got really neurotic about cleaning everything with Lysol wipes when we were going through this... For my son this lasted about 4-5 days; for me, I had this for 8 days! (Right after I had my second baby... talk about easy weightloss plan!)
Anyway, regarding diet, our pediatrician said to feed my son anything he would eat, the Banana, Rice, Applesauce, and Toast diet didn't make a difference. So, peanut butter and jelly sandwiches, yogurt, etc are all fair game. I know that when I was ill, really nothing sounded interesting, so I think the point is simply to make sure your son keeps hydrated and gets as many nutrients as he is interested in.
I hope your son feels better and the rest of your family stays healthy!