You have a lovely horse!
He is on the forehand however, and I would not be asking him for extended work until he is able to carry himself more over his haunches - too much extended work even on a horse that is in self carriage puts a huge amount of straight on the legs.
He is not moving his back at all - all of the 'length' is coming from his back.
I'd love to see you in a better fitting saddle as the poster above said, with significantly longer stirrups so that you are able to better influence him through your seat. Then do a lot of work on increasing his bend, developing a soft, swinging back through riding millions of transitions, leg yield, and changes of rein. Then start asking for a few lengthened strides at a time.