My keet started talking probably around 5-7 months. I didn't do anything special to get him to talk, I would just talk to him often. If you look up talking budgies on YouTube (watch disco and kiwi) you'll notice a lot of them like to say baby bird so that's what I started calling him. I figured if he'd learn to talk baby bird would be an easy phrase for him and that's the first thing he learned. I could hear him practicing and it was probably about a week or two before he could say it really well. They tend to like b and p words, so pretty bird, baby bird, stuff like that is easier for them. I think he can still learn to talk even though he hears the birds outside. Watch some videos and try talking to your keet in a similar way to how the birds in the videos talk. They like when you use a crazy/ high pitched voice. I try to imitate disco when I talk to my keet.
The fact that he stops to listen when you talk is a really good sign. My keet will be babbling away but as soon as I go near the cage to try to get a video he stops and waits for me to talk instead. My keet and I have a "talking spot" lol unfortunately he doesn't come out of his cage yet but whenever I get off the couch he will run to the corner of the cage and wait for me there. He knows that when I start putting my sneakers on for work I'm coming to talk to him soon so as soon as I start getting my sneakers on he goes to our spot and babbles until I go over to talk to him. I put my face up against his cage and he loves to be right near my mouth. Sometimes I can actually get him a couple inches out of his cage just because he wants to be so close to my face. He will rest his beak on my nose and start falling asleep as I talk to him
try talking to him before he's about to go to sleep. If you see him getting ready for nap don't disturb him but try to quietly talk to him as he falls asleep. Have a talking session with him before you put him to bed, they seem to really process things while they sleep. Sometimes my keet will go from practice jibberish to clear words just over night.
I hope he'll talk for you, it's literally one of the greatest things ever, I could sit and listen to my keet talk all day. Good luck!