My experience has been that hydration really is the most important. If he has a hard time keeping fluids down, try giving him a tablespoon at a time every 5 minutes or so and try Gatorade. There's something about Gatorade that helps, I think maybe the sodium. If he has a fever and gets dehydrated, the dehydration can cause the fever to spike which then adds to dehydration.
I hope he's feeling better soon. Good luck!