Most horses will find one rein more difficult to work on than the other. The rein you find that he falls in on is his stiffer side and by falling in he is avoiding having to hold himself balanced and correct.
Use the lunge whip pointed at his shoulder and give him a flick on the shouldler when he tries to do it. Also take a step towards him sending him out using your voice at the same time. Try not to let the line get slack as he will also see that as an opportunity to drop his shoulder and fall in.
Hope this helps.
