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!