Hey! On my horse, he is exactly as yours, I wasn't being able to do the flying changes, I started to make leg yelding, pushing him to the outside rein and then asking the flying change mostly with my outside leg!
Now I just keep him engaged to my outside rein and my outside leg keeps him active in the hind legs!
Not sure if you can understand what I'm trying to say... it's not easy to explain!
Hope it helps
Poles, circles and ,,8’’s may help
You can try to do it over the poles or small jumps! Might be easiest to do it in a collected canter! You can begin training trotting over poles so he begins to think about where he steps and so he is just more careful with where he puts his body