If there is anything in your yard he might have gotten into including lawn chemicals, mushrooms, leftover fat from a cook out etc then that might be the issue. If he likes to chase and catch bugs a swallowed bee can cause this sort of upset too.
If he is holding down the water you can see if some plain pedialyte will help him feel a little better.
Because he is so lethargic you may want to consider seeing the emergency vet today. If he was just doing some vomiting but was up and around that would be different. But the being 'flat' and not even willing to get up for a drink sounds to me like he may need some supportive care. If you can take his temp rectally normal in dogs is 101-102 F. You can read directions here on how to take a dog's temperature
http://www.peteducation.com/article.cfm?cls=2&cat=1670&articleid=1082
if its up over 103 then I'd definitely suggest seeing the vet.
Hope this helps you!
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