I think being his friend is the best you can do. You've talked to him about the subject, so he probably knows you're someone who will listen and care and not judge him.
It might be that your friend doesn't have an eating disorder. I think it depends if there is a fear there, if he's not eating because of emotional reasons or he really has a lack of hunger. I know some people who don't eat much and are thin but there's no fear or control or negative emotions attatched to food, they just don't feel hungry much.. this is possible.

Just keep an eye on what your friend says and how he behaves around food. If he has an ed at least you've put the idea in his head and he might eventually accept he has a problem and seek help for it.

You're a good friend to care