Ignoring the question of head set/position for a minute, is it possible to go in a round outline under a western saddle?

My horses worst fault is that he will, given ahfl a chance drop onto the forehand and it affects his back muscles so its important for me to be able to get him working through at 'normal' paces before jogging etc.

My horse will ( if ridden well) go fairly quickly into a nice rounded outline in an english treed saddle. He then tracks up from behind and lifts his forehand. If worked forward and through you can release the reins and he will take them down into a passable western style head carriage.

If I ride him in my western treeless saddle it is lot more difficult to get him to round up and work from behind. instead he tends to drag himself along on the forehand (except hacking out today where I was damn grateful for the fork to grab when we did airs above the ground pre galloping!)

I wonder if the different western position for me affects him, or if its the fact my western is treeless? I dont want to change back to english as my horse seems to cope better with western aids.

I also really like this saddle as I feel safe and can stop spooks more quickly (essential as I have to hack up some really busy main roads)as I can feel his back tense before it happens - in a treed its usually happened and Iv hit the deck before I notice whats going on!

Yes you can, and it's a useful exercise if you've got a horse that's naturally a bit on the forehand. We do want the horse to be low in its neck, especially for pleasure, but it also needs to be able to do this with its weight on the quarters and the forehand light - a big ask.
After an easy warm up,including a lope, take the reins a bit shorter and ride the horse up in to the contact, squeezing with the legs rather than bumping. You'll also need to keep the hands a bit higher than usual. If the horse resists keep at it, and keep the leg on. You should get a shorter, rounder stride - try it at the jog first. When you release the contact, the horse should stay light even as he lengthens his neck, and should not rush off. If he does, go back to the shorter contact and legs on. Try to get a small improvement each time, but make the sessions short as it can be tiring to the horse.
But remember, the aim is to have the horse soft through the back and slightly flexed at the poll on a very light contact - short and round is only a meansto an end.