First of all, trying to force a 17 year old to drink water, as another poster suggested, is one of the most ridiculous things I've ever heard. If you try that, you're just going to needlessly cause problems.
What you want to try to focus on is helping your son get enough fluid intake each day with the least amount of sugar and other unhealthy additives. Since he is having difficulty with his stomach, I would first see if there are any unsweetened herbal teas that he may like. There are some that are known to help settle your stomach such as chamomile, raspberry tea, mint tea and ginger tea. You can even combine these to get different tastes. Another tea he may like is green tea (again, preferably unsweetened). Green tea has been found to have numerous health benefits, but it does have caffeine. If he can't stand the teas without them being sweetened, try to use honey instead of processed sugar.
One more thing. As strange as this may sound, if he is having stomach problems, drinking water alone may actually make him feel worse. This is certainly not the norm, but if this is a problem for him, make sure you talk to his doctor about it. He or she may have some suggestions on what else he should try to drink.