Stomach bugs usually don't have fevers (it's really really rare for them to).
If you can't get fluids into him, you'll probably need to take him to the ER for an IV. Usually that's not necessary. Slow sips until they can start keeping down larger ones. Just make sure you're familiar with the signs of dehydration so you can bring him in if necessary.
I'll bet your problem with the juice is that he gulped it / drank a lot more than he was of the water. The body naturally fights drinking water when we have the stomach flu, because our electrolytes are already out of wack, but *dives* for the sugars in juice (which it desperately needs for brain function). To "ease" into it, try making juice popsicles, OR give him some suckers to get a little sugar into him so that his body is gulping at the juice.
Soda (sprite/7up typically) & gatorade/pedialyte are the best rehydration options short of an IV (because all 3 have salts and sugars, which the body needs). Shake the soda a bit so that the carbonation doesn't act like a propellant. It doesn't need to be totally flat (pedialyte and gatroade are both essentially flat soda with more kinds of salts than most sodas). Later, straight chicken broth (just drain the solids out of chicken soup and put the broth in a sippy cup) can get the fats and proteins into him that he needs for manufacturing antibodies. I personally skip BRAT and go straight to McDonalds ((there is no better sick food on the planet that McDonalds hamburgers and Sprite -or in my case, coke, for the caffeine, but I'm adhd) but McD food is practically predigested and loaded with fats... it's exactly what a sick body needs once it can keep food down, because it requires minimum digestion for maximum absorption... of course the same reasons why it's bad for us when we're healthy)).