If your horse bends to the outside, you've too bend him on the other side, but I think it's just a temporary solution. If I was you I would like to work on inside line of the riding area to improve the straightness of your horse !
Try doing shoulder in excersises to get the horse to understand that you are asking him to move his head to the inside. Also do some circles and make sure you can get a bend in his head and neck to the side you want.
You might want to look into the fit of your saddle as well or other signs/causes of discomfort. It may be un comfortable or hurt to bend in the direction with that specific lead. If you roundpen your horse does he still exhibit the same behavior?
It's when you use a lunging surcingle (a surcingle is like a girth but it doesn't just go on the stomach it goes on the back too as a training aid) and put a lunge line through one of the rings on the inside and it's like lunging but it's making the horse flex to the inside. Talk with your trainer or look it up online I bet it sounds really confusing when I explain it 😅