I myself am now a lefty I used to shoot right but I'm left eye is dominent so I taught myself to shoot left handed. It could be a couple of things I'm no expert but some of the things that I went through were a release is much smoother with a release aid rather than fingers, finger shooting takes a lot more practice because if your release is not smooth you can torque the string a get a bad arrow flight. try a release aid. next check his dominent eye just because he is left handed doesn't mean his left eye is dominent, other than that I had to take my bow into a pro shop and get it tuned and get his form checked to make sure it is consistent, sorry if that doesn't help those are just a few things I went through shooting right handed than going to left hended. Good Luck