If only there was a huge televised event the majority of the country watches coming up and the president typically did a broadcast interview right before it....
Like, especially in the leadup to an election and when people have doubts of his mental ability, now would be a great time for that.
They keep saying he's fine, but he's also only appearing in public for short speeches/comments where he still gets confused. If he's fine, show everyone he's fine.
Have him do an actual interview where he needs to think on he's feet