Our 2-year old son, Thomas, is just getting over a horrible stomach bug - probably the same one as your son has. We had the worst time getting him to drink anything. We called the doctor over the weekend and were told we SHOULD NOT give him Gatorade. The doc recommended the pedialyte, (Thomas wouldn't drink it and he wouldn't eat the pedialyte frozen pops), or white grape juice, diluted with around 50% water. He also advised against giving apple juice, although he said it would be okay to give him 'a little' milk.
In the end the only thing Thomas would drink was water - if it helps, we got him to start drinking it by singing that awful Wiggles song, (Gulp-gulp, drink some water...).
Thomas only threw up a couple of times, but he had the diarrhea for 5-days.
I do hope your little one feels better soon. It's just miserable when they are so sick.