If you can sit it okay, what we do is give them a time and a place it's acceptable to be goofy, like when it's just us on the trail. My instructor's daughter (who's actually my instructor now)'s horse, Shy, loves to kick up her heels when she feels good. My instructor gives her a time and a place, "play time", when it's okay to act that way, and Shy understands when they're "working" it's unacceptable.
I think Sonny said what I would probably do, pull him down to a trot as soon as he acts that way. That way he learns if he acts that way, he doesn't get to canter with you. Hoovey can get the same way in a canter, he's just so excited to feel good and share it with me.
"Sit tall in the saddle, hold your head up high. Keep your eyes fixed where the trail meets the sky. And live like you ain't afraid to die...don't be scared, jut enjoy the ride." - Chris LeDoux