If you're concerned with him being dehydrated anything you can get into him will help. (except for maybe pop, it's actually a diuretic (sp?) and milk can upset little stomachs if they are sick) But juice, gatorade, pedialyte -types, all will help with that. Or if he's eating anything, a little bit of peeled graped, or peeled apple will help too, because of the juices, and shouldn't be too harsh on his little tummy.
My kids never liked the taste of pedialyte, but they loved the generic store brand from king soopers - known in other parts of the country as krogers or dillons. Especially the grape flavor.
My pediatrician always told me not to give the kids anything at all within the hour following them throwing up. Something about giving their tummies time to settle or something... can't remember, but it did seem to cut down the times they throw up when sick.
I hope he feels better soon!!