baseballdownsouth, if the purification level would be adequate, you might want to look into one of the Brita-type pitchers (Pur makes a big one designed for the fridge)for your son's water --- cheaper, less to store, no trips to the shops for water. My son uses one quite happily.
If your son will have a kitchen, you have additional lessons to teach, beyond budgeting, bank account handling, and Laundry 101. Start now with lessons on Grocery Shopping And The Value of Those Flyers By the Door, Food Handling, and Cooking: Basic and Adventurous. Don't assume he knows anything...and cover hand washing a lot. It's great fun to cook with your son, in any case.
I made up a personalized cook book for him, with the recipes for his favorites, some easy recipes, and some reminders about safe food handling and the value of noticing Use By dates. He now has the rep of making the best spaghetti sauce in the baseball ghetto....even though it means he now knows that's where I hide the vegetables!
Encourage him to approach the food situation with his roomies early. Are they sharing or does everybody have their own.
FYI, I'm finding that, with groceries and gas (obviously entertainment and misc are there for all of them), my son spends $50-$75 a week.
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From 'Nice Guys Finish Last' by Leo Durocher:
Baseball lives at the center of a never-flagging whirl of irreconcilable opinions.