You will probably find that in order to maintain the circle you are pulling him out with your outside rein instead of almost leg yielding him out with your inside leg and getting that nice bend around your leg.
In the meantime I would get his teeth/back and saddle all checked incase he is setting himself in that position to evade discomfort.
Definitely agree with some with some good lessons . You need to feel you can push him out onto the circle from your leg, rather than pulling him out to the circle using your reins