Well you can "block him" by using a wall or whistle sit him before he arrives, but the best thing to do is teach him to slow down and heel properly. Many assume a young, fast dog will simply come to heel spontaneously, but it is an OB, yard "thing". When they won't or can't, something they should know wasn't taught.
The first position is a front sit wearing an OB lead (6') with a bumper in his mouth. Use the lead and teach (pull) the dog to swing around (fluid arm motion) as you step back on the side he is supposed to heel/sit on. Then step forward and finish (easy, relaxed and under control). When he is proficient at this (up close and not one lesson), move the dog to a remote front sit at the end of the 6' lead (with a bumper in his mouth) and repeat, here, "swing", step forward and deliver. Finally, move to a checkcord, increase the distance and continue reinforcing the new expectation adding in the speed factor.
The step forward and hand to your side (sans checkcord) are physical cues to slow down, pivot and sit. Eventually, physical cues are minimized or eliminated (especially the step forward and back).