Yup, I got some thoughts.
1) Don't bother.
2) Tracfone has its issues and its place. Not sure this is it.
3) Don't bother with 3G GSM phones at this point, T-Mobile's already decommissioned theirs, and AT&T is twilighting the network in a little over a year, however I think going VoLTE capable feature phone might be the best route.
4) Boost, post Sprint divestiture and going to Dish... plus what T-Mobile is doing to Sprint's network as part of the merger will leave this a bit of a dead-end sooner than later. A lot's going to be happening on this end, and it's gonna cost money with new handsets sooner than later.
Here's my suggestion:
There's not really PAYGO plans much anymore outside of Tracfone, and it sounds like the ability to neuter internet access might be handy from your Pop's perspective. Look into US Mobile. No per line base fees per month like Ting, just custom prepaid monthly plans for relatively cheap on either T-Mobile or Verizon's network. He could do 75 minutes, 300 SMS messages and no data for $5/month plus tax.
They also have some no-name Android flip phone on sale through the end of the month in their store for $30. The form factor alone might be enough of a disincentive for much internet usage, and Android does have some parental controls anyway. If your father's interested, he could grab one himself and switch over at the same time and potentially save even more money over what he's spending with Boost, then he can have control of both accounts and manage things with a single login.
https://www.usmobile.com/