No, it was
never a Verizon phone. It is a GSM device that used to be carried by both Cingular (before they assumed the AT&T name) as a contract phone, and by Firefly, themselves as a pay-as-you-go device.
Firefly phones, which were marketed as a simple communications device for young kids, used to be available at big-box stores such as Target,
Walmart, etc. At one time, you could buy the airtime cards off the same rack as Tracfone, and others.
As for the phones, they are still around, but are now available only at the
Firefly website.
Given the details you've shared about your grandson, I think the Firefly would be an excellent first choice as there's not a lot of "stuff" to memorize (OP's situation is very much unlike yours, MandM-Mom) -- it's a few, simple buttons.