My startup crossed $2M/yr and now co-founder is going for a power grab
So 3 years ago, me and 4 guys start a company. Roles are as follow:
CEO
COO
Chief Development Officer
Chief UX officer (CUXO)
Chief Data officer (Me)

Everyone is 20% owner.

Currently my team has 9 people.
Chief UX officer team is 3 people.
Total company is about 17 people.

Our business model is basically...we collect publicly available data, clean, normalize, and enriched with other data sources. The data is made available via an API as well as front end.

I basically architected the entire backend infrastructure that manages all of our data pipelines as well as was significantly involved in development of the models/algorithms for enriching data. Chief Development Officer helped out with some execution in early stages.

Chief UX officer was responsible for front end. He also manages the API operations after we publish data to the db he manages.

During formation, ~3 years ago, we agreed no one would have the role of CTO. About a year or so after formation, CUXO said he wanted his title to be changed to CTO.
Chief Dev Officer and I strongly opposed this and with the support of CEO, we decided that there was no need to change things from what we had originally agreed on.

Chief Development Officer left the company about a year ago and all his responsibilities fell on me. Though tbh, he wasn't doing much since I had hired a team of offshore engineers.

(Today)

CUXO is the last person to quit his job and go full time. We had a meeting this morning and he said that He wants to change his title to CTO when he starts next month.
His stated reasoning was pretty silly to be honest, He said that he wanted the title because it would be better for his career after the startup - which to me is probably the least compelling reason possible.

CEO and COO said that didn't want to get involved and it was for me and CUXO to sort it out ourselves.

I'm planning on having a meeting 1:1 with him this weekend and explaining that I think the role of CTO implies a set of responsibilities far broader than would be appropriate for either of us since we've kept a very sharp line between front end and back end in terms of management. I think him being CTO will also cause confusion with regards to our roles in the company, both internally with our respective teams and externally when we have client meetings.

I also feel like I've built an extremely sophisticated back end system on a pretty unbelievable budget...and having him as the CTO I feel would massively undermine the work that I've done.

Lastly, I don't like the idea of us trying to go back and change the original agreement. We all decided on equal equity (20%) as well as no CTO because we wanted to make sure everyone felt equal.

Aside from the fact that I don't think he deserves the role of CTO, I really don't like the idea of us re-negotiating the terms of the business formation. Suppose he gets the CTO role, will he later come back and argue for more equity?

I should also mention that the first time he asked to be CTO, he said he deserved it because he setup the slack and gmail instances so that makes him CTO. This indicates to me that he is fundamentally unrealistic in his outlook. Also combined with the fact that our relationship has always been a bit tense, I expect him to react very negatively to me opposing his title change.

Another important piece of information is that the CEO and CUXO are old friends which I think might be impacting his desire to avoid getting involved in this.

I often read about startups failing because of disagreements with co-founders, and I'm really concerned that this could become a much bigger deal than it should be depending on how insistent the CUXO is. I strongly object to him taking this role and I feel like he strongly feels he deserves it.

Does anyone have any advice they can give on how I should proceed?