while I agree about sitting back a bit, don't get so hung up on the idea of "sitting on your pockets" that you end up hunching over, or making your hind end a pile driver into his haunches.
your seat is already about right, so just a bit more of the upper body coming back will help, IMO. also, while you are feeling this horse out, and riding through tall vegitation, I can understand that you aren't putting much leg on him. but, you may find that by sitting up a bit more, and actually putting MORE leg on him, you will get him to do less of that falling in on his shoulder sort of turns. it will take experimentation , but honestly, I'd be very happy to sit as well as you do.