I think talking with him about why he wants to go to one session a week would be useful. I don't think it's fair he dropped that at the last minute although perhaps he wanted you to think on it and see if you came to your own conclusions.

I saw my T for two times a week during a very very painful time. We probably did less therapy and more just support during that period. It was rarely working through things and more me sobbing uncontrollably. Once I found proper medical treatment, I've now gone to weekly again. If I were going twice a week now, I'd say it wouldn't be helpful and he might not think or realize that it's helpful to you.

I would think about why you're going twice a week and outline the reasons you find it beneficial. His goal is to help you and if he's concerned that twice a week isn't helpful then perhaps he's trying to get you to a place where he thinks therapy is effective.