I believe my son my have an eating disorder, but what can I do?

I can't get him to speak to me. A few days ago I told him about my concerns, he denied them, and now after he comes home from school he just locks himself in his room and never comes out. He's avoiding me completely. He's 6' 2'' and he looks like he can't weigh any more than 105 pounds...He's painfully thin. I know that he used to be 145 only a few months ago. He seemed very happy before but is a perfectionist. I want to go up there and talk to him but I have no idea what I can say. I don't want to make things worse. I'm worried I might be a part of his problem. Either that, or his new girlfriend. She is also painfully thin and I think maybe she may have an eating disorder and might have pulled him into it. And no, I am not basing this belief off of his weight loss alone. I caught him purging one Sunday and he also eats very little. One of his friends was worried about my son and he told me that he hadn't been eating at school.

*might have

It's awesome that you've noticed this now instead of later. I know that no one discovered my ED until years after I had started.
I know that when my mother found my stash of laxatives, diet pills, thinspo, and other crap I freaked out. I avoided her like the plague. I didn't want to get confronted by her and I was determined to keep restricting/ purging.
Please, don't be worried about getting him upset. It'll pass eventually. It's important that he gets help asap. I didn't and I ended up overdosing on pain/sleeping pills about a month after my mother found everything.
Just try to use "I" statements, (I know this sounds corny, but this is what they teach us in treatment centers.) like "I am concerned for you, because of this, this, and this" or something like that. Also, it'd be helpful if you do this at a time where the two of you aren't busy/tense/angry/etc.

EDIT: A very important thing is that if you're going to take him to the doctor, then you have to make sure the doctor knows what they're doing.
Mine was so clueless that I was able to convince her that I was "perfectly healthy and had good self awareness". Her own words.

What a great parent you are to notice the change and to want to help. You really don't have any choice but to drag him to a doctor, not only to help with the eating disorder he might have, but he likely suffers from depression too. Maybe a med for the depression will result in him eating again.

Good luck to you.

That is a very sad disease and is a mental problem. My aunt is anorexic. There is more than just not eating, you can become compulsive and not speak to family as well. He needs to see a doctor. It might be hard to get him to go if he is not talking to you. There is really no cure for this, you just need to help him.

You may want to take a look at this informational site, http://www.eatingdisorder.com/eating-dis... on there it has a lot of helpful and useful information about different types of eating disorders, their symptoms and causes, treatment options that are available, and more.

Also there is a toll-free helpline, 1-800-714-8354, which you can call and speak to a trained professional and explain your sons situation. They will be able to advise you on what steps to take with your son, and what you should be looking for.

Good Luck..