Dear [CTO],
usually, you have set up our recurring meetings. Regarding the meeting of the longterm planning team, do you wish to organize that or shall I do that?
Best wishes,
...
From the group of CTO, PM and Developer, to me it seems quite appropriate for the PM to organize a planning meeting. The CTO is usually more in the role of a consultant and/or stakeholder. However, if he is very involved in the product in your case, he might be interested to organize such meetings. Or not.
Just ask him nicely in a way expressing your interest in helping him and the product. I do understand that the developer appears to be pushing you, however if this is the most effective way to organize the meeting, then take the active role!
Also, inviting to a meeting implies some responsibility that a certain goal is reached within that meeting by moderating it. Dev probably doesn't want to take that responsibility, maybe for your own good (he doesn't want to take something from your role), maybe for your bad (he doesn't want to have the work).