Have person behind him poke him right in the middle of his butt with a show stick. It doesn't take much force to get them to move forward.
But before doing that go back to the most basic things and try to provide incentives rather than pain. Make sure you are providing relief to him if he does the right thing. Start with just getting him to give his head to your gentle tug on the lead rope and stop tugging the instant he gives his head. You may have to do that 50 times, but once he gives rather than resists, then that is the first step. Once he can do that well, get him to take one step forward with the minimum pressure on the lead rope and let him relax for a minute and scratch him between the shoulder blades for a few seconds as a reward. Then ask for 2 steps and so on. I can't think of a calf we have not been able to break to lead in a day or two (actually in about an hour total using 10 minute sessions spread out over a day or two) using this method.