I think that is more than generous. What do you spend on groceries now? Divide that by the number of family members and you will get a rough idea of how much he will need. But if you think about it, the grocery bill you have includes things he won't be using. I'm thinking household cleaners and other specialized items that are really for one or two other members of the family. You can acutally figure out an average between now and the time he leaves. I would also suggest going shopping with him to really show him how to live within the constraints of a budget. You can blow through even 600.00 a month if you aren't smart about how to shop. But 300.00 for one person will keep him well fed and I think even provide a little extra for special meals and nights out. I don't know your S but if this is his first experience budgeting it may be easier to dole out his monthly food $ every other week until he gets used to it. It may keep him from running out early.
Another kind of fun thing that you may want to think about is teaching him to make the basics: browning ground beef for mexican or italian dishes, marinating and broiling chicken, pork and beef, assembling a healthy green salad and making an omelette. That kind of thing will serve him well in the dorm kitchen. You could even get fancy and take a class together. A rice cooker is a cool thing to have as is a water boiler if they are allowed in the dorm rooms. It's handy not to have to use the whole kitchen for quick meals.
Sorry for running on...I know I got all teachery on you
