This is a disrespect issue and one I had to deal with when I first began training my mare. If he comes in to you, get the end of your rope and get to his shoulder. With the rope, swing it at his shoulder so it 'drives' him away. As soon as he takes a step away and leaves you, reward him by immediately stopping swinging the rope. He will undoubtedly test you to see if you are serious about him coming in on you, so this will have to be repeated continuously until he finally understands that coming into your space results in an irritating hit on the shoulder with the rope.
Make sure you:
- do not move away from him. If you do, he is dominating you and disrespecting your space, as a stallion would do to control a herd and become a 'higher horse' in the herd. By moving away from his pressure you are teaching him he is the leader.
- release pressure IMMEDIATELY when he moves away from you. Release of pressure is his reward. If released too soon or too late, your horse will become confused and will react.
- are not confusing him by being in front of his drive line. His drive line is his shoulder. If you stand in front of this, this signals to your horse to turn in or away from you. Stay behind it to 'drive' him forward.
It's useful to assert your leadership before lunging in a roundpen. It is safer - you are not connected to the horse with a line - and ensures the horse moves his feet when you ask him to.
The drive line is behind the withers so if you stand in a line with him level to behind his withers you should be able to push him on foreword with your whip. You may have to get after him a few times and then he will get the hang of it. You just have to be quick on your feet and watch him closely to be able to anticipate when he will try to come into you so you can correct him before he makes a mistake.