Hi Maria! Welcome to the forums!
I know exactly what you mean with wanting to make your birds feel as comfortable as possible. I have a little female keet and you won't believe all the effort I put in to make her happy and healthy.
3 weeks is not that long so maybe you should give them more time. As long as he's eating, drinking and pooping normally he's probably ok, at least physically. Keets are shy and maybe it just takes longer for him to get used to his new home. Also from experience I've seen that keets have different personalities (or keetonalities
) and maybe you just have a more introverted bird in your home.
But it's not a bad idea to visit a vet, just to make sure that your keets are in good health. That's what's recommended you do with new pets anyway. Then you could use the opportunity to ask about the male keets behavior.
I'd love to see some pics btw.