I've been anorexic since I was about 11, it was very serious until I was in my mid-teens. Anorexia is something that doesn't exactly go away so since then I've gone back and forth. But I haven't been in serious trouble for several years now. I really don't think anorexia is something that ever goes away, you just have to fight with it if it rears it's ugly head.
Don't gang up on him, it is the fucking worst. Do not ask him why he's not eating in a room full of people. It will make him lash out and likely cut you out. The best you can do is be a supportive friend, ask him how other things are. Don't badger. If you feel the need to tell him how you feel. Express concern to him in private, not in front of other people. Honestly, you can't really force him to do much, getting in contact with a professional for further advice is probably the best, but ultimately it's like an alcoholic, they have to make that decision to get better.
His sources could be from a lot of things, desire for control, conscious self harm, etc. sometimes it has very little to do with genuinely wanting to be a stick thin person. When I was extremely ill the thing I wanted most was for someone to notice, and express to me that they noticed and they cared. But during the years leading up to that point I hated people commenting on it, because it was my business. Um this is probably not making a lot of sense because it's a close issue and I just woke up oops.
Basically, be a supportive friend, just being around other people can really help, make him aware that you care about him without chastising him, get in contact with a professional who you can disclose more to and will be more apt to know the course of action in this situation.