I'm assuming he is bulging his shoulder out? Be strong with the outside leg and tighten up a tad on your outside leg. If he still doesn't listen use a whip or spurs to keep him from bulging. There is nothing wrong with using spurs. When my mare gets lazy and starts dropping her inside shoulder I stick them on, a few pokes to say "get off my inside leg" and we are done with that lessons for at least 6 months... Better then kicking her in my opinion.
It could also be a balance issue? He has been under saddle less then a year and turning, especially at the canter, may just be plain hard for him. Try gradual turns, like starting on a large circle then spiraling inward to the size he has trouble making. Posted via Mobile Device