I have a really lazy horse?

I am an experienced rider but with a really lazy horse! He doesnt listen to your leg. When he first arrived he was a nervous of everything but now if is almost boom proof! His jumping has really come on! I have recently been not able to ride him as much because my yard has changed hands and another person is really interested in riding him. She is not that experienced and he gets away with everything. His aids and transitions have gone down hill and his respect for leg is bad aswell. he also is very unbalanced and stiff around corners. Can anyone suggest what i sh ould or can do about this???