YY to what nahaudin says. Highlight This on his accommodation application form. They might even put him in with other students who have dietary requirements. It doesn't matter if they are different ones, most people who have one are very understanding of other people's needs.
You have totally made the right decision about SC with tuition from mum over the summer.once you get used to it it's easier that you think. He just needs to plan ahead more that other kids, as you can't just reach into the cupboard and pull out a packet of biscuits if you are peckish at 2am.
He will needs to learn to cook some basics things like sauce for pasta and mild chillie and soups and store portions on the freezer.then he can make easy meals with GF pasta and packets of rice that microwave in 2 mins.
It's easier if you start again from scratch when you think of meals , rather that start with existing diet and try to buy everything GF. Remember most food is naturally GF - all meat fish, dairy, fruits and veg, pulses and beans. Many grains.
He needs to learn what to eat for breakfast and Lunch. Dinners are actually the easiest.
E.g. If breakfast is normally 3 slices of toast, he will need to eat 6 Gf slices to get the same amount. So have 2 slices of GF toast and scrambled eggs make in MW or baked beans or boiled egg and a tomato. Or porridge with Gf oats and some fruit.
He needs to practice GF lunches , he will need to make this each day as he won't be able to pick up Gf sandwiches on campus. Even if they say they do them, they mean " we sometimes have them in on some days but it's only one packet and we are usually sold out by 10am ".
GF wraps are good for packed lunches.
Somecrisps are GF, he needs to learn to read the labels for what flavour / brand.
Most chocolate bars are ok.
Ditto yoghurts.
One of the good things about many kids with ASD if that they don't mind eating the same thing every day. This will help with the GF diet 