Drinking water is good. Have you offered food? Did he eat it?
I would watch him one more day. If he doesn't vomit, eats and drinks normally, then I wouldn't rush off to the vet. If he has normal poos and pees, that is a good sign, too.
If he continues to vomit, doesn't poo or has a hard time pooing (or something weird is in it), then I would ring the vet. If he goes for more than 24 hours without eating or drinking, call the vet. If he vomits anything weird, call the vet.
Otherwise, he probably just has an upset tummy and will come out of it. My beagle does this all the time.

If he continues to be lethargic for more than a few days, then I would call the vet.
(I used to be a vet tech and a zookeeper for YEARS and these are the general things that are good tip offs for needing vet help)