I am looking for suggestions as to a T.V. option for my elderly, mentally handicapped uncle?

AKMonger: Use Echo or Echo Dot. Even so, he may require assistance since the Echo products are newer technology.

Can he talk? Alexa can operate a fire TV. Or any kind of smart device like Alexa. Just say.... Alexa turn ____ TV to channel 4

The radio and a hired companion for walks - reading and puzzles.

Simplest one is best. See if you can find one with an "on/off" button or switch and a dial to change channels.

He will get confused and be unable to learn any new thing, no matter how simple it is. He just can't retain the information.

I agree with Grandma1954 that you should "outsource" the control of his tv. My MIL is in LTC and has a tv in her room which the staff turns on and off for her. Her dementia is fairly mild, with short-term memory loss and she can't even use the pre-programmed push-button phone that has our photos on it (really large).

Find out what programs he likes to watch (and none that are scary, dark or negative) then put that channel info on the tv so the NH staff can find it easily.

I think Echo or Echo Dot can control the TV with turning it on and changing channels.
The question is will your uncle be able to use the voice commands necessary to use the devices?
If you know what shows he wants to watch there might be a way YOU can control from your phone.
I am wondering would it not be easier, maybe better for the staff to get him into a common area where he can watch TV and be with other residents and not isolated in his room? (unless of course he is bedbound then getting him to a common room would be more difficult)