Try lounging him where you have a corner in your fencing so he can't pull away, it also helps him with his balance, actually that is why round pens are so popular because of how the horses use the fence line to balance. In my riding ring all my fence corners are rounded so when I lounge I can use these corners.

Yes, he will eventually learn to allow you to have more contact with his mouth. As a dressage rider (I think that is what you liked to do) it's nice to have the right contact with the mouth were if you close your fingers the horse learns to get into a better frame instead of on the forehand or hollowing out their back. You could also use a sursingle when you lounge eventually so he gets used to carrying himself in a better frame. But, first he needs to learn how to move forward and lounge. Keep walking towards his hind and pulling him inward at the same time, it's ok to start at just the walk. You probably know all this.

There are probably a lot of videos you can find if you do a search that may offer some tips you may not have thought of too.

Sitting trot can definitely be a workout for the rider. Yes, if a horse is broke western he may be confused by posting. Try posting for a few minutes then doing sitting trot, then post, then sitting trot until you can do the posting longer and longer.

Western horses don't have very strong hind ends, they are ridden long and low with a loose reign and tend to be on the forehand. It's hard when they go like that to find their balance when a rider posts. If you can get him lounging and then add in a sursingle where you can gradually get his head up in a better position where he learns to use his back and hind end better, that will help him build up muscle first without a rider. Then you can slowly ride him little by little so he builds up the muscle and hind end to also better carry a rider. You can keep him in better shape by just lounging in the surcingle on the days you don't feel like riding.

Also, before you ride him from the ground but have the bit in his mouth, take the bit and pull it back gently so he can feel it and open and close his mouth. This will also help him learn to feel the bit and then relax. So you will be pulling the bit back "gently" , he will open his mouth, then relax the bit so he is more relaxed when he feels the bit.