Gym is a luxury. If he wants it, he gets a job to pay for it.
Mobile phone - look at buying a refurbished handset (if he doesn't have one already) and get a cheap contract (Tesco do some for less than £10 a month, I think). Music Magpie is good for refurbished handsets.
Food - get him a "cooking on a budget" cookbook. Once he makes friends, can they cook together to save money? Eg it's cheap to make a load of spag Bol, but not that useful if you're cooking for 1 and don't have freezer space for the leftovers, but if he has 3 friends and they take it in turns then it would work out cheaper.
Course materials - wait until he starts and then see whether he needs much. My uni course recommended several textbooks, but none were actually essential. The uni may give access to online materials too. And he may be able to buy books secondhand (and sell them on afterwards).
Other than that, he just needs to be really strict with spending. He may think he needs a fancy laptop, new clothes every few weeks etc, but he really doesn't. A lot of students when I was at uni insisted that they needed Mac books etc - they really didn't; I had a second-hand Dell and it was more than enough (unless he needs to run specialist software etc).