My horse used to do that and I would put more weight in the outside stirrup, or just make sure you are balanced. I caused my horse to duck in and cut corners because i used to lave a problem of leaning in.
Make sure you have a good connection between your inside leg and outside rein. Practice going straight first and gradually turning rather than trying to turn right off the bat. This is most likely a problem with balance. Make sure you balance yourself and he will feel the need to balance under you.
I would say try to block with your inside leg and push him into the corner a bit. You can also use your inside rein to help guide/bend him around your leg and through the turn. You can also try stepping into your outside stirrup after you fully complete your jump (which should help counteract him from dropping his shoulder).