Because Henry is a stray and has been outside for quite some time, he will need time to adjust being inside. His meowing is due to the fact that he wants to go outside. I hope you have purchased some kind of squirt bottle as this is what you will have to use when he starts his meowing to go out.
Like others have said, give it time and be very patient with him as it is a big adjustment for him.

In the summer, I let my cats out on the gallery, supervised, and I have a baby gate, so they can't get downstairs. I live on the second floor. Mooky is the one who mostly loves to be outside. I have let him out a few times, when we had some nice weather. He even meows at the door or at the window because he wants to go outside. All I do is give him a couple of squirts of the bottle and tell him firmly, "NO" and he stops.

I wish you all the best of luck with Henry and I pray that gradually, Cubby and he will become friends and bless you for taking in Henry and caring for him.