I researched this once before for a similar situation: spec-wise this is one of the best you can go with right now, even has band 71 and a charging dock:

https://www.walmart.com/ip/Schok-Classic-Flip-Unlocked-GSM-Phone-Black-Blue-Red/235330763 (easy return to walmart if you dont like it)

If not that one, then an alternative is the NUU 4FL https://www.amazon.com/NUU-Mobile-F4L-Flip-Phone/dp/B0858QDP5C but it doesn't have band 71 and specs are a little lower but both work.

In the end, I was able to convert him to using a moto e6 using the biggest possible font and ui sizes, locking the icons via nova launcher (so they cant be dragged off) and only putting a couple big icons on there, one for contacts, and one for a shortcut to the person he calls the most.