If there is an art to getting horses to load in a trailer, then this it.
The first thing that must happen is communication between horse and his handler. The horse must understand what is required of him and handler must make it easy to understand.
Thus, you must start by getting horse to move forward in either direction. After a direction is established then get him moving in opposite direction. A round pen is excellent for this purpose.
When horse gets good at moving both directions then it's time to hook a lead rope to his halter and lunge him left and right. For instance, when you point left, horse must know to go to your left. And vice versa for going right.
The final result should look like this. You're holding lead rope while facing your horse. As you hold lead rope in your right hand, you hook middle of lead rope with your left thumb. Lift it up and point to left.
Your horse, now knowing what to do, immediately goes to your left. After a circle or two you switch hands and get horse to go to your right.
When he gets good at that, then send (lunge) him through gate openings, barn door openings, etc. Also, lunge him in smaller spaces to help him deal with his natural claustrophobia. You can do this by lunging him between you and a fence. As he goes back and forth make space between you and fence smaller. Be careful not to get stepped on.