Use a lot of outside rein and inside leg!!
sit up and look straight! if he decides to dodge the corner just keep looking straight and use all your leg power to keep him going straight through the corner etc
I'm sorry, I didn't specify that he drops his shoulder and dives around corners on a jump course.
Do shoulder in, leg yield off tge track and on the track, ride squares and make him engage his shoulders more to pick them up and if he is ignoring you give him a tap with a long whip, be firmer if u have to.
Collect a bit in the corners so he doesn’t drop his shoulder?