Hi there, Rollbacks should always be 'into' the wall. That keeps them shorter and more on their haunches/hind legs.
Also--Are you using skid boots? If he gets sore, he may not want to put weight on those skinned places.
Another couple things that really strengthen the hind quarters and will help chest/brisket area, too, are:
1-work in deep, soft dirt - 20-30 min at a jog trot and about 10 min at a slow canter - if circling split time so you are going both ways equally
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2-back him up - when working him in an arena, stop every 1/4 round or so and back 15-20 feet. Backing will also keep his head lower so he more balanced for athletic work.
3-Work him in figure 8s, both at a jog trot and at a canter. If he doesn't do flying changes, either work on that or pull him up to get correct lead.
Flying changes can be accomplished with a hard shift of weight to the direction you want to go in and of course, reining him that way at the same time. (You probably already do flying changes just wanted to make sure you knew what I was talking about)
Also make sure you are doing equal turns on your roll backs or one side will develop better than the other.
Also on a barrel horse make sure he learns to walk the course, jog trot it and slow canter it to practice on the 'correct' angle he needs to come in for the hard turn around the barrel.
If you have any problems with his head coming too far up when working, I would work him in draw reins - reins attached to cinch ring and ran up through the round hole in the bit. A western snaffle with a 6" shank is good for this. This is something you can do as you notice him flipping nose or throwing head to far up to turn well.
Sometimes just a good longe line with the draw reins tied and dropped over the saddle horn works for this, too. Just longe him for 10 min each way. Do give him enough slack to work - just not enough to flip his nose or throw head up.
Another thing to remember - if he gets sour ears or switchy tailed he may be sore. . or he may be bored.. so you always want to do a bit of outside trail riding or pleasure riding to keep his mind fresh.
Hope this helps! Good luck!