To build the muscles necessary for self carriage...get out of arena, take horse out into fields, woods, or dirt road, and hack. Walk up every gentle slope you can find. Don't let him canter up, that's him cheating. In a few weeks time you will have a well built machine back there. You'll know he's got the proper muscles developed when he voluntarily walks down hills dead straight by sitting his back end rather than weaving back and forth. Warning...horse will be forward and having lots of fun. Rider too.
You just have to see your distance...you don't have to like it.