We originally applied for an O-A visa but for some reason were granted a non-immigration O visa. (we didn't realize it at the time, and that caused some reporting problems and $$$, but is another story).

   The question now is what category of non-O visa extension can we apply for? We can qualify an extension on retirement basis since the docs for the O-A visa had been focused on financial issues. But we also can, and would prefer, to apply on basis of visiting son who has residence permit. 

   Thanks for your help, George

You can apply for a extension of stay based upon being a member of your son's family it he has permanent residency.

See number 19 here. https://www.immigration.go.th/en/?p=14714