My horse wont get round, help

I ride a 12 yr. halflinger gelding that is 15.1 h. He is perfect! Lead changes, easy to sit to, bu there is one thing. He is impossible to get round. The only thing to do is use draw reins but it doesnt help because I mostly need him to be round in shows and he doesnt. He also bucked A LOT when he was younger. He even bucked me off and gave me a laserated spleen! lol So anyway, sice he bucked, his mouth was pulled on a lot and now he was a "hard mouth" So I cant just move the bit. What can I do to get him rounder? He uses a kimberwick already

A bit really does nothing to get your horse round. If you get the body round, the head, neck, and jaw easily follow.

Baby steps first: how does he move off your leg? The slighest pressure and he should b moving that way- you should be able to push him over w/ your leg without any effort. Once that is established, you can start working on rounding.

1st: try this at a walk and master it before moving up to a trot. hold your reins at a bit tighter level than he is used to. You want to hold that position throughout the ride- do not give in to him trying to yank them out of your hands- make sure your elbows stay bent and your shoulders back. Now, once you are holding his head there and not giving him anything, try pushing him more forward with your seat. It will be very bouncy and choppy (not wanting to use his back/free it up) until he starts to stretch through his neck and back instead of just his head and neck. You should see the muscle working all the way down his neck, not just in the upper section. It is easier to loosen a horse up if you make him do those bouncy strides (while holding him) in the beginning when doing the exercises. If he starts to relax and give into the reins, let him walk looser.