Most Helpful Girl
You tell him like you just told us. He is YOUR date, somebody YOU said yes to, and something YOU agreed to. You need to decide if this guy is actually worth dating if you feel that your eating disorder is that bad. You want to be with people you trust. And if he is somebody you want to trust, then you have to trust him to be somebody who will not get upset for your change of plans. However, do you have another plan for a date?
Most Helpful Guy
Just say you'd really like to do something else and suggest it.
Personally. I would rather a date yells me the full truth. I would not hold it against her.