My mare is a pacer also. She was mostly trained to canter when I got her, but she would slip back
SO luckily, I only had to work from the saddle.
I would ask for the canter and if she would pace I would bring her back down to a trot (by posting because your horse follows you.)
By bringing her down to the trot, she would learn that pacing was no fun. This is because trotting takes more muscles and is more tiring than a nice canter.
Also, once he understands cantering, let him get used to cantering and get more balanced. Then work on making it more "flexible" do circles! Nice big ones!!
Oh and just a though, I think teaching him in a round pen would be a little easier, since going in a circle is easier than an oval
^working on circles helped my mare's canter a LOT