Should I apply for my son's Non-ED visa now or enter Thailand as a dependent and apply later?

Unless you're going to have the documentation from the school he's enrolled in to apply for the ED visa at the consulate in your country before you come here, he can't get an ED visa now.

He wouldn't be able to come in 'dependent on your visa' 😕 He'll get in based on the nationality of his passport (they don't have a "stamp kids in just because the parents get in" sort of thing here).

I'd say if you can't get the documentation from the school to apply for the ED visa before you come at the very least get a single entry tourist visa for him from the thai consulate in your country before you come here.

That will get them 60 days when they enter, which should give you plenty of time to get the documentation from the school and apply for the 90 day ED visa at the immigration office in the area where you live.