Be a true friend to him. He needs a lot of love and understanding right now. People who have eating disorders have issues with control. They can't control everything around them but they can control what goes into their mouth. It is a very serious situation if he is truly does have it.
There is a tell tale sign besides being very thin. Look at his arms and if he has very fine thick hair on his arms then you can bet he is. Your body protects itself by growing the extra hair. I had a couple of friends with it and one eventually took her life. The other friend was the sister who studied with me. I remember when she was going through it at 40 plus years of age which is not the normal, I let our circle of friends know what I suspected. I was doing a dieting program with her and she went too far and I knew what she weighed and what they were saying at the program. When I told our friends they poo pooed it and thought I was over reacting. Eventually it did some damage to her heart.
If you are good friends with his parents I would be inclined to let them know there is help for him. If I was not seeing something as dangerous as this I would want a friend to tell me and not hide it. They can be very good at hiding it by wearing big clothes and other things.
I hope he gets the help he needs.