It is very common for a horse to confuse the canter and half-pass aids, particularly in early stages of learning half-pass. It is very important that you don't reprimand your horse for this as he will lose confidence in what you are asking and this can result in problems in both canter and half-pass.
The aids are similar but have small, very significant differences. In canter, the rider's outside leg comes back a little, the inside seat bone should lighten to allow horse's shoulder to lift, and the rider keeps their shoulders straight. These small movements in your upper body are significant aids to your horse.
In half-pass, your body position is different; your shoulders should turn towards the direction of the movement, your outside leg comes slightly further back than for the canter aid, and you should close your body towards the inside elbow on each stride. This will take your body weight towards the inside in the correct direction for the half-pass.
Consistent replication of the aids is paramount so you horse has clarity and you can help your horse more by asking for half-pass and canter in different places in the school.
Diane Followell Classical Dressage Trainer