Can I transfer my son's Non-Immigrant O Visa to an Education Visa in Thailand?

Totally possible. You can do it two ways,

you can wait until this yearly extension is expiring (when you apply for a new extension) and change the reason for next years extension to education rather than one based on being the child of someone who legally works here

OR..

You can go with paperwork and cancel the extension of stay based on being the child of someone that works here and then as soon as that extension is canceled you apply for a NEW yearly extension based on education.

I'd say go to your immigration office and ask them what documentation is required to do each of the things I outlined and then go from there.

If your employment is secure, is there any benefit of transferring him to an Ed visa, rather than remaining as a dependent piggy-backing on your work extension?

You can not transfer the Non Immigrant O visa based on dependant into a Non Immigrant O visa Education visa. You can ask the school and Immigration if it is possible to change when you apply for an Extension of Stay using based on education.