Congrats on trailer! That is awesome. I learned a trick about harder to load horses years ago and it has always been successful for us. You would be best suited to have 2 people. You need a lunge line or long lead and a lead rope. Get the horse as close to the trailer as you can with him acting calmly and where he will stand quietly. Have the lunge line/long line attached to his halter. The line needs to be going where you want the horse to go and the best result is to wrap it once around some part of the frame of the trailer, have one person hold this on the outside of the trailer but somewhere where they can see where the horse is. You should try to avoid any pressure from this line on his halter, it is only there to prevent any backward motion. The lack of pressure here means that he won't resist. Have other person ask the horse for forward motion in to the trailer. The closer he gets have the person on the long line take up the slack, but never allow pressure. So the closer he gets, the better and he never gets further away. If he pulls on the line, keep resistance so that he does not move back while the person beside him asks for forward motion and if necessary with light taps on his him/bottom for the knotted end of the lead line, do this until he stops resisting, be insistant that he move forward off the line and once he steps forward there is no pressure. If he is standing there and checking out the trailer and being cooperative, give him time, let him do this. But if he is not interested or is paying attention to something else, etc ask him to move forward with the pressure from the rear - light taps from lead rope, etc. Once he moves forward at all let him inspect again, until he gets bored, etc. I know that this sounds long and involved but it has never taken more than 30 minutes for us to get one loaded this way. I am sure that there are the equine exceptions though! Basically you want to pressure from behind, kindly and not from his head. If you give him time each time he moves forward - with praise and rest, it will be a lot easier the next time. Just remember to make sure he is paying attention and that the long line only has any pressure on it if he is hauling butt backwards, the key is to not allow backwards motion and always encourage forward motion, the long line person must be diligent in keeping out the slack. Best of luck!!! Hope this helps you!