Lost of serpentines, and make sure you use inside leg and get them on a nice correct bend.
Firstly, make sure it's not something you're doing in your body or seat. All my horses used to throw their bums in on the right because my seat was skew. If it's not you, carrot stretches to the right where you make him touch his nose to the area where his girth would sit, every day for 2 months should really help. 20 on the stiff side, 5 on the regular side.
What my trainer has me do is 2/3 of my lessons on my horses bad side, and 1/3 on his good side. It's been helping just getting him used to it. Good luck!
My horse is the same. Just that the reins and do small circles and keep making them small every time you ride.