make sure while cantering that you aren't posting. That usually inhibits the horse to canter. What I do, for my guy, is I'll give a "cluck" or a "kiss" and then give him a little squeeze with my legs. If he doesn't canter and just speeds up, I'll slow him down for I don't want him to learn that the cue means to do an extended trot. Then I try again, this time I'll "cluck" or "kiss" and then give him a slight kick.
Also what I've seen also, in some horses (YES I know this is the wrong way, but it might be worth a shot to get your gelding used to cues) that if you are trotting, and then lean up into jumping position or somewhat in jumping position they'll take off in a canter...at least mine does.
Don't be too afraid to use the crop. The crop is a re-enforcement of your legs. Ask for the canter, say "canter" (if that's what you say to get him to go) and then give him a slight kick with your legs. If he doesn't go, slow him down into a regular paced trot, then ask again, this time kicking just a tad bit harder. If he still won't go ask for the canter and then kick him a slight tap with the whip...and keep getting harder until he goes.
He might be testing you...I had a horse that wouldn't canter for me and I'd keep trying to canter her, but she would only speed up...so finally I gave up and just trotted around. Finally I got so annoyed that I kept at it until she cantered.
Your horse might just be lazy and not like to canter, so he could be giving up a fight