Jocelyn, I know this isn't really what you asked about, but wanted to share my experience…
The one and only time my CAGs wings were trimmed was by the place where I got her before I brought her home. A year and a half later, she was still missing feathers.
I mentioned this to my avian vet and he made a note of each flight feather on her wings & which was missing, so we could compare at a later date to see if they were growing back in.
We were wanting to figure out in the mean time if she was breaking them off or if the person who clipped them did a poor job and basically cut them so far back that they couldn't grow back in.
Since there was a chance that she was doing it, I wanted to give her a chance to use her wings and enjoy the feel of flying. In my mind, I thought that it might distract her from breaking them, if that was indeed what she was doing.
(When we got her, she was able to fly, but lost the ability to do so.)
I started working with her by first taking her out and holding her on my hand. I would say "fly" while slowly moving her downward so that she would flap her wings.
Eventually, she would flap them every time I took her out and just said the word "fly".
I also said "fly" every time she vigorously flapped her wings in the cage.
Then, I started standing very close to the cage and enticing her with a favorite treat.
Every time she would take off (very poorly I must say), I would say "fly".
This served as a dual purpose. I was hoping that if she ever became flighted and accidentally got outside, maybe by giving her the command she would come to me.
Anyways after the course of many months, slowly moving further away, giving her the command & rewarding her with a treat, I can happily say that not only has she regrown those feathers but she also flies to me on command and will fly back to her cage. (Now the problem is her flying to just anyone & landing on their heads!)
Hope this helps! I know that it made me very sad to know she couldn't fly. I hope that yours regains his ability as well.
(Note: In this post, I am just describing how I helped my CAG learn to fly again. I wasn't trying to teach her free flying and recall in the outdoors.)