I agree with those that say to go slow and reward small steps. There is no telling how many calves have been ruined with calves and tractors because it wasn't done right. Yes, it can work, but it's an admission that you don't know what you're doing frankly. If you start with a step, you'll have a great calf that will have bonded with your son and most likely if he gets spooked at a show will simply move closer to him.
I saw a heifer that had been broke with a four-wheeler and she was terrified of them. At a show where they delivered extra bedding with one, she went absolutely crazy and broke her tie ring and didn't stop until she got to the perimeter fence of the facility, and she bounced off of the chain link once or twice as well.