Specific exercises that will both strengthen and make him more receptive to your individual legs are what is needed here.
Exercise one..... Shoulder in. This is to introduce the legs in controling his body position and where he places his legs.
Exercise two....Turn on the hind....or pirouette. This can be considered as the follow through to the shoulder in as you will allow him to turn farther around and allow you as the rider to "feel" where the horses legs are moving under you.
To attain the right lead I prefer to ask from the walk and not the trot as it often ends up as a running gait that becomes unbalanced and then ends up as an unbalanced canter.
To ask from the walk attempt this on the left lead so you understand the feel. On the turn on the hind you will in the beginning allow some enlarging of the turn on the hind legs so they actually do a very small circle ( 4-6 meters) and you will also allow some small lateral sideways movement. What is imparative is that you will be a left or right rider ( depending on the lead). This means you will sit more heavily on the side of the lead you wish. You will also ask with the heavier weighted inside hip,leg at the moment the the outside hind leg has moved over and about to land inward under the horse.
If you time your aids correctly you should get an instant canter and it is imperative you REMAIN weighted on the lead side. I would plan this last exercise in the middle of the arena to allow forward movement into the canter. The next step is to reverse and try with the right lead.