I agree with Crimson on this one. The only thing I can think of off the top of my head would just be extra work on the bad side to balance things out. He sounds undermuscled on his "good" side too. The fact that he tries to canter indicates that he's trying to get out of the trot (it's hard work) or he's speedy. Muscle work will help the former. Lots of circles in all gaits. Do more walking on the bad side, as walking builds muscles. Do not pull harder on the reins, he doesn't know how to make sharp turns yet.
Take this with a grain of salt...I'm a little uneducated.
"The wise man thinks he knows nothing.
The fool thinks he knows everything."