Aww hun, we've got gastro-entiritus (sp?) going round here at the minute. It sounds like Rhys has something similar.
My friends little girl (14 months) had it last week and the doctor told her off for taking her to the surgery. He said that there was nothing he could give her to ease it, she just needed to get it out of her system, and by going to the surgery, she'll have passed it on.
If you're worried, ring the surgery and ask their advice.

The GP may be able to come to you for a home visit.
As the others have said though, just sips of water and lots of sleep.
Hope he feels better soon.

