I think this is probably the best approach, however I find it hard to believe there is a possible title that he could suggest that wouldn't similarly undermine me somehow.
I'm trying to avoid the conversation turning into an argument about whose done what for the company because I think it would irreparably harm our already tense working relationship, however I think without a doubt I have overwhelmingly been responsible for the companies success from a CTO perspective, especially since taking over the responsibilities of the former Chief Dev Officer. So much so, during client calls I'm often introduced as the CTO by our CEO. CUXO has had practically zero client interactions since he has so far not been working full time for the company.