First if your horse lunges do that,fully tacked. Every horse is one sided, just like people [left/right] If he takes the correct lead each way it is a rider problem. If its a rider position problem you'll need the help of a ground person to get you straight. If still wrong, then it is a horse balance problem.
Canter departs are developed progressively.
A green horse canters from trot, using a corner, let him look to the outside bring your right inside hand away from the withers and use the outside leg and a kissing sound. This frees all weight off the inside front leg making it easy for him to take that lead. It is clear and simple for a green or unschooled horse BUT it produces an outside bend and crooked canter.
Next still using the corner. Add the direct rein pull of the inside right rein and outside leg. The leg activates the canter and the rein indicates lead and asks the horse to look in that direction.
Last, Prepare as above may be done from walk/trot in corner or on straight line.
Ask for slight bend in the direction of the lead you want. Use your inside leg and rein to create a bend and your outside leg to ask for canter.
Remember the better your timing and balance the easier it will be for you and your horse to be partners. It just takes time and practice. Also do both sides evenly or his good side will become his bad side.