Ask him where he got his information since he didn't sail on the Dawn Treader.

WWYDI you heard someone quite old claim to have went to Narnia?

Knowing me, I would hear them out. True, they could be lying and they could be mad or they could be telling the truth.

WWYDI you and siblings were the Kings and Queens of Narnia? (if you don't have siblings, you can think of three different friends)

WWYDI you and siblings were the Kings and Queens of Narnia? (if you don't have siblings, you can think of three different friends)

Fear for the state of the nation, as my siblings are the very definition of mischief and snark ...

WWYDI a king of Narnia seemed to appear in front of you, in our world and in need of help, as Tirian did in The Last Battle?

Be scared out of my mind at first. Since I'm afraid of anything ghost like.
WWYDI You were told you could visit Narnia for only five minutes.

Hope I would be as positive about a mere five minutes as Caspian was (though he has the advantage of no longer wanting wrong things). And, if I can, take a camera!

WWYDI, while stargazing in our world, you saw two heavenly bodies pass Tarva-and-Alambil-style?

Depending which heavenly bodies and when in history I lived, I might schedule a caravan to take some gifts to Bethlehem.

WWYDI you found a pool with a reflection of a beautiful woman in it?

Knowing that I'm a single man, lose my head for a second or two. Then wonder by what power or magic do I see this reflection?

WWYDI a faun suddenly appeared infront of you?

Well, I'd probably do this: https://youtu.be/w6BDaLVhP1U

WWYDI you met a courteous Mouse?

I would invite the mouse to come meet all my TLC friends.

WWYDI you were asked to show Puddleglum around your town for a week?

Take him to a couple of landmarks and to a couple other other sites known during current events, and try to listen well enough to glean from his perspective. (A more somber answer, sorry.)

WWYDI Uncle Andrew asked to borrow £5?

I would say no, and offer to help him set up a lemonade stand to earn a bit of money.

WWYDI you discovered that there were talking animals hiding out somewhere in our world?

Look for them or at lest look if the source was reliable.
WWYDI you saw Aslan?

Kneel and listen to what he has to say.

WWYDI you found out that Narnia was real.

Be extremely annoyed that I haven't managed to get there yet. 😆

WWYDI you did get to go to Narnia but then no one believed you when you told them?

I guess I would just kind of shrug and go on believing I'd been there anyway.

WWYDI you found Susan's horn at a thrift store?