Shoulder-in trouble?

I'm riding a Thoroughbred gelding in his early 20's for my upcoming freestyle dressage performance. He's been a lesson horse for most of those years, so he knows the shoulder-in well. However, it is ME who doesn't know how to work him properly. I can get a reasonable shoulder-in on the rail tracking left, but I can't do the same when I come off the rail into the dressage boundary. The horse is an ex-racehorse, and he has a bone shaft in his front right leg, so it's even harder for me to get a shoulder-in tracking right because he doesn't like that side and drifts off the boundary. I thought about using a dressage whip, but it's hard enough to get a nice, slow, relaxed trot from him anyway without the whip hanging in the corner of his eye. He tends to take the shoulder-in commands as a signal to go faster anyway. Any tips?