Plenty of people on my course and in my halls older than me (at 18) when I went 10 years ago. Was in the minority obviously but certainly not alone.
He will find people similar to him and with shared interests if he explores joining societies (there's literally one for everything you could imagine: ballroom dancing, law, religion, yoga, shooting, beer making/drinking, circus skills, reading, poetry) along with societies for each subject. Id highly recommend him joining a society around something he is already interested in but also trying something completely new and random. My best friends from uni are the people I met in societies rather than halls or on my course. Still close friends 10 years on.
Halls yes traditionally will be filled with partying 18 year olds. But not every one in every flat will be like that and if that's not what he's into he will find people somewhere with shared interests. Even if that's just a quieter flat or group of people to go the cinema or the library with.
The beauty of uni is there is so many people from every walk of life with every type of personality and want all in one place.
My advice is for him to go with an open mind, be friendly and sociable (where he feels able) and push himself to try new things. Even if it's just the once! I met my husband in the society too!
Best of luck to him and hope hen enjoys it.