My heart aches for you and your son... As a mom of a son (22yrs old) I can feel the pain that you are feeling.. It is easier for us Mom's to be sick than to watch our children be sick... Eating Disorders happen... they just happen...
It is not something that is caused... (just my opinion).. or because someone didn't do something.. so the best thing is to take it really gentle on yourself.

I know that your son can beat this.... with support, he can do this..

I am 3 meals a day, every day. This is my mircle from God,
in answer to my prayers and others. If you could have seen me even 6 weeks ago..

So I know this can be done.. I offer my understanding and any help, support that I can give to your son and you.

Sincerely,
freewill