You say that he's got little to no training, so I'd think that he hasn't built up enough muscels and hasn't got the balance to carry a rider properly. I'd suppose you start right from the beginning and do lots of lunge work, esp. To let him canter at the lunge line - as this is where he's got a problem. Lunge work is good to build up a horse's muscels.
After some time you can start to ride him again at a walk and trot, if he hasn't got any problems with that, to improve his balance with a rider on his back.
Maybe it would also help you to try not to sit the canter but rather unburden his back and stand up slightly (sorry, but I don't know the right word for it in english, hope you understand it anyway).
I don't know how good a rider you are, but it might also be you or your seat disturbing him somehow and bucking is his way of telling you.
Best thing you can always do is getting a good trainer and let him help you and your horse. Because a trainer can see it live, as everyone at the forum can only guess.