I think that if you want help from him with your relapse it would be good to tell him.

I think his desire to help you would be much stronger than his disappointment. I don't understand why you think you'll get better help from him by omitting important information rather than telling the truth. I don't think you or anyone else can impression manage a T in the way you think.

It seems a distortion to me for you to believe that your planned approach is the best way to get the help you need. There is no way to test this belief and prove yourself wrong and then try a different approach. I'd encourage you to consider just being straight up with him. Bulimia is a dangerous behavior that carries serious risks for your short term and long term health.