Cantering on the lunge is quite hard work, so they do need to be pretty fit! Is he lacking fitness perhaps, or balance? Try and keep him on as big a circle as possible, so aim to walk a circle or oval to allow him a bit more space. It's also better to make sure you bring him back to trot yourself, before he breaks and not let him make the 'decision'. Gradually you should be able to ask for a bit more.
I know a lot of people who just don't canter on the lunge though, as it is high intensity work on usually quite a small circle.
Agree with Rhino, it is quite a lot of strain on the muscles and legs to canter on the lunge if they aren't fit.
I am currently working on getting my mare more forward on the lunge, and have literally just been doing walk and trot transitions for a few weeks now with side reins on. I keep the sessions short - 15 mins in total now.
I also praise her a lot even if she gives a momentary bit of good work. I am finding she is gradually getting better now!