it’s not conformation because he stretches his legs behind him on purpose, while racking up he stands normally
if he is doing it at a stand still, it's conformation. you cannot change that.
You can use poles or if you have access to lunging equipment use that; such as side reins or a Passoa training system might help. Each horse is different.
My mare liked to trot like she was 2 horses as well!! POLES POLES POLES!! My gal is bigger and has a large stride. We set poles about 4.5 feet apart for trot. keep a VERY cadenced, steady pace, even a little slow. It will encourage him to LIFT his back legs. This is something that is most likely a strength issue, so it will take time.