How much liquid are you letting him try at a time? Our pediatrician's online tips suggest:
"For a child over 1 year old with vomiting (but no diarrhea), the best fluid is water or ice chips because water can be directly absorbed across the stomach wall. If your child is 2 years old or older water is best, but half-strength lemon-lime soda or Popsicles are also okay. Stir the soda until no fizz remains (the bubbles inflate the stomach and increase the chances of continued vomiting).
Start with 1 teaspoon (5 ml) to 1 tablespoon (15 ml) of the clear fluid, depending on your child's age, every 5 minutes. After 4 hours without vomiting, double the amount each hour. If your child vomits using this treatment, rest the stomach completely for 1 hour and then start over but with smaller amounts. This one-swallow-at-a-time spoonfed approach rarely fails."
We used this tactic the last time we had a stomach bug in the household, and it worked for us, although by the end of the first 4 hours, my daughter was VERY tired of only being allowed to have a small sip at once.
Best wishes!