A multi entry visa is pretty much useless at the moment with 14 day quarantine. He needs to change it from a visa to an extension of stay by showing 400k THB in a Thai bank account for 2 months. The 90 day report is for those who already have a year long extension of stay, and have to report to immigration every 90 days.
You don't have a visa. You have a year long extension of stay with a multiple re-entry permit. When does your passport expire? If you have enough time, use your current passport and extension to get the COE and enter Thailand. Get the stamp moved to your new passport when you arrive. Continue with your yearly extensions......But if not, and you don't have enough time, then get a new passport, enter with both, and then get the stamp gransferred over in thailand. There is also nothing stopping you from entering as a tourist and then getting a NON O in Thailand, and then continue with your yearly extensions after that. But it would be a waste of your current extension.
Marrying your partner will of course get you a year extension of stay based on marriage, but if she is the mother of your biological child, it will also allow a year of stay based on your child as well. Just for info.
So you have never made an extension of stay after your initial OA? Only entered before it finished and got admitted for a year? I may be wrong, but I thought you had to make at least one extension of stay based on the same reason as your initial visa (retirement), before you can change to extend your stay based on marriage. In which case you will need insurance for the first extension of stay. You can change to an extension of stay, based on marriage, later (in a year) with no need for the mandated insurance. Someone can correct me if I am wrong.