I just interviewed (for an article) a woman who earned her USDF Silver Medal this year (a really big national, lifetime dressage accomplishment) riding a Mustang. She told me the one thing that made the biggest difference to the horse’s topline was using an AquaTred lap-pool type treadmill, which we have access to locally. You don’t say where you are but if you have access to that type of facility it could be worth trying. This lady RAVES about it. Cost was reasonable, too - something like $35 for first session and $25 each after that.
In addition to what has been mentioned, you will need to be patient. He has along back, which will lend itself to weakness. Long trots with him stretching forward (in 2-point, if you can manage), gentle work up hills, etc, will all slowly help.
As he fills out you will need to continually re-evaluate his saddle fit and adjust as you go.
Enjoy him 
