First off, your horse looks absolutely fantastic. I would have never in a million years pegged him as a 24 year old. Good job on keeping him in good condition.
Before I say anything, I'd like to state that I haven't exactly read through all the responses you've gotten. (I'm quite lazy, forgive me!) If I'm repeating anything that's been said, I apologize.
The biggest area for improvement I see is your hands. Your horse is going with his mouth open, meaning he's quite uncomfortable with the bit in his mouth. This can be due to or in part because of the bit that's in his mouth, from your pictures I can't tell if that's a contributing factor. What I can tell you is your hands are causing him discomfort here and are a huge factor in your having difficulty engaging his hindquarters. Without significantly softer hands, you won't have a comfortable, responsive horse. You're also quite stiff through your arms and shoulders. I did read that you're having some issues with your shoulders. I don't know what your limits are when it comes to rotating your shoulders, but loosening up as much as possible is a must throughout your whole upper body. Once you become more elastic and your horse has more freedom with his head and neck, you'll be able to ask him to engage his hindend more effectively. Essentially he's unable to bring his hindquarters underneath him right now because he's too restricted in his front end and cannot box himself up any more than he already is (which is not only uncomfortable but is simply bad for the soundness of his body.) Basically, take your hands out of your lap, give your horse his head, and spend some time simply moving forward and keeping your body nice and relaxed.
I do like your leg. Besides your heel slipping up, which compromises your balance and the effectiveness of your leg, I really like where it is behind the girth. It's in just the right place for flat work. Basically, work on dropping weight into your heels.
As for strengthing his hind end, there's nothing better then hill work. If you have any hills on the property he's on, spend lots and lots of time walking and trotting up them. And no two-point. Sit right down on his back to increase the effort he has to take to push up the hills with his hind end. Riding him through combinations is also a good way to increase his strength.