What visa options are available for my 12-year-old son when I retire in Thailand?

You would have him come into the country on a single entry tourist visa, that you get from the thai embassy online before you come here

That will stamp him in for 60 days (and you can get one 30 day extension on that too at the immigration office for 1900baht)

Once he's here get him enrolled in a school, get the documentation from them to apply for the ED visa for him inside the country at the immigration office.