Yes, he has lost a great deal of ground and will have to make that up somehow. The college course is probably something like a computing coding course to get his foot in the door but he will need a specially tailored programme. College doesn't mean the same in the UK as the US and some college courses here provide basic skills. He survived off the Harry Potter box set when he was away which will have helped to shape his world view - never give up no matter how hard it gets, be a good person etc. If he needs more specialist help, it wouldn't surprise me if JK Rowling stepped in at some point. Her organisation Lumos does wonderful work for children worldwide.
There are various access courses pre university - hopefully things will work out for him. At the moment, he's enjoying being normal, going to watch Manchester United play and such things.