OK, I am a strong believer in using crops and whips where necessary, but I don't believe this is a necessary case.
The horse may be unfit, and unbalanced in canter, therefore making it very hard for him to keep his canter. Hitting him and yelling at him to go probably wont help the situation any as he will still be unbalanced, and unable to keep the canter properly, and will maybe even begin to resent you asking for the canter.
Also the over use of whips can make horses less responsive and nappy, also may cause bucking. What I would do is do plenty of bending exercises, in the form of serpentines and 10 meter circles and such, to build up muscle and flexibility.
Also, he sounds like he runs in and out of canter. Does he do this or is it just me presuming things again
If he does, he will become unbalanced going into the canter as it will be more or a flat run-into-it transition rather than a nice bouncy smooth transition. If he runs, i would suggest collecting him up using your reins and seat going into canter, and keeping a steady medium trot before and after. This can be got by half halting (taking gentle tugs on your outside rein) when he starts to rush.
Of course, if he doesnt run into it, this is useless lol. Oh and keep riding him through the canter, pushing with your seat and legs to keep the momentum going. I hope all this wasnt a load of poo
XxXxX