Forget Olympia and Kensington, they're too far out of the centre of things. He will be exhausted before he gets to anywhere. Islington isn't much better either, well it does have some good restaurant options not too far away. Euston is nearer in, but it still seems to me to be next to nothing.

Because of the minimum fare, and if looks visibly "senior citizen" there won't be a problem getting a black taxi from Paddington to Metropole. Don't get a cab or private hire / Uber etc, with one important exception: you may find a driver who will be happy to do several trips for you spread over a few days and will take the rough with the smooth. So both Metropole and Paddington would seem to work for you, Metropole West Wing would be my preference, and it's well located overall. I personally haven't had a bad experience at Paddington, though I do think it is somewhat overpriced whenever I look at booking there.

Of the others, have a good consideration of DT Hyde Park. it is central, there will be places he can get to on the doorstep, notably the park itself, and plenty of restaurants nearby.

DT Marble Arch (which I don't think I've stayed at yet) is extremely well placed, he'll be able to get to Oxford Street's shops on his own steam. It's location is fantastic, and less than 10 minutes by taxi from Paddington.

Experiment - while you are there - with him on the London buses. He will always be able to get a seat and he may enjoy it.