B.B.
Keep offering the liquids... If he'll drink it give it to him... BUT if he is throwing up right after drinking then YOU have to limit the amount he drinks at one time... One teaspoon, then if he keeps it down about 10 minutes later 2 tsp... so on and so on...
Put out a variety of crackers (not heavily flavored ones) so if he wants one bite he can just grab one when he wants it. My 4 yr old had the 24 hour flu and loved crackers with cheese spread on them. It was the only thing he'd keep down. I know its not the healthiest sick food but does he like sugar cookies? He may be more open to taking a bite of a cookie if your really worried about his not eating.
As long as he is putting liquids in and is not acting lethargic or out of sorts don't worry about him going lite on his food. His belly is feeling yucky and he may be saving you a lot of cleaning up "big spits" with chunks. :-)
Keep offering and if you feel that mommy sense go off, then take him to the dr. You will probibly be told that you just have to wait it out...