What should I do with my colt who when trotting he gets excited and races ahead then bucks and rears and plays up?

I love round pen and hot walker work. The round pen gives them time to play and be silly and the walker teaches them respect without you having to getting angry or tired. They can try to trot ahead without successfully getting away with it. When they trot next to their rope it gives them the release/reward.
I second the chin chain. If he's bright, you may only have to do it once or twice for him to realize it creates pressure when he does the wrong thing and releases when he does the right thing.
He should respect you as his handler if he is excited and hyper or not. Lunging is just a bandaid to the problem IMO. Try doing lots of ground work starting from the beginning. Take your time with this too. It is the foundation for your riding work.

If you're looking for a quick answer though, I would stop and immediately back him a fair amount when he does this. Bucking in hand is unacceptable at any age.
Try lunging him before doing showmanship. If all else fails use a chain connected under the halter and to the side upper ring of the halter. I have to do this with my quarter horse who thinks he can pull everyone around
try lunging him before you go to lead him. Hopefully that'll get his excitement out! :)
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