Are you trying to find a reason to convince him, or to convince yourself, or both? It seems like you have some very good, legitimate concerns with the way he wishes to do things. So in my mind it feels like you want to convince him so that he will be on board with things, instead of trying to sabatoge them?

I'm dealing with some issues with an alter that is very negative and likes to tell me how horrible I am, why I fail at 'everything', why no one loves me (when in fact they do), and what I should be doing, etc etc. My t is working with me to realize WHY he acts in that way. I know he hates when we feel happy, successful, and most strongly when I feel hopeful. In his mind, feeling good, doing well, being good, and being hopeful, are all the ways in which I set myself up for a fall. This is because when my abusers could sense that I was feeling hopeful, they would prey on that hope. The fall from feel good to feeling rotten is much greater than feeling low to feeling rotten. Likewise, with another alter, if people can't get close to me, they can't hurt me, so she will lash out at anyone who seems untrustworthy. They may have worked at the time these alters were created, but they are causing me a lot of pain at the moment. I've gotten the lashing out on one board by explaining to her how thankful I was for keeping me safe at the time that I needed it, and actually BEING thankful. I was finally able to admit to myself that I couldln't have handled life without her at the time. She is much more willing now to work with me, when I can see how she was necessary for me. There are still issues, of course, but it's much better. With the really negative alter, I still don't know how to deal with it. I can see how HE thinks he is necessary, but I am not ready to admit that he was ever essential to my wellbeing, when I feel so horrible whenever he is around. And I am still having issues with him, and worry about him taking control. My t always says that it is so important for me to learn to trust my parts, and understand them. Sometimes it takes the heart a lot longer to realize something than the brain.

I dont' know if that gives you some ideas. But if there were a way for you to create some sort of alliance, or at the very least respect of certain boundaries, you may find things easier. Have you mentioned the issue to your t yet? (you said he was out the entire last session, so I'm wondering you even had a chance). If not, could you write a letter, or email your t?
Wishing you a peaceful resolution!!
xoxoxo
IJ