Me personally, I would put him back into a snaffle until you figure out exactly what he is going to do at the canter. At least that way, if he decides to blow up, you can get his head around to the side without hurting him. What I do with a young horse the first time cantering is I will keep them in a long trot for quite some time so that they are tired and lacking a bit of air (makes them less willing to act silly). Then I will just start urging them to go a bit faster and faster. Don't try to force the canter, just encourage and encourage until they pick it up on their own. They all will, sometimes it just takes longer than others. Once you get him cantering, I would just let him canter in large circles until he relaxes. It may take quite a while and it might happen quickly, it all depends on him. You should be able to feel it when he flattens out and relaxes. For the first canter, I will push them to keep going until I feel that and then I will cool them out and quit for the day.
One other thing, since you don't really know how he will react, I really suggest that you keep the inside rein significantly shorter than the outside rein so that you don't have to re-adjust before getting control of his head if he acts silly. Also, make sure you have a good hold on something so that he won't be able to buck you off easy. That is, of course, if he does decide to buck and not just kinda act squirrelly for a minute.