Keeping calm is very important and giving him time to look, smell derstand what's in front of him! First I would try treats or food, try to convice him to get in!
If he gets close you can try to put one of his front legs on the trailer so he knows it doesn't kill him!
If he still doesn't want to go you can try to put a lounge around his hind legs and slowly pull him a little bit more, don't let him go backwards!
It can take hour, but be pacient and persistent
Hope it helps
Stay calm, be patient, and give your horse time. Sometimes horses develop an aversion to trailering because they experienced a bad ride in a trailer, or simply because they had a bad experience that they somehow relate to the trailer. I had a mare who slipped while loading her on a rainy show day and we had trouble with her ever since.
You really just need to focus on your horse. Don't try to force it in the trailer if it's not ready. Just work with it using positive reinforcement like feed or treats. Additionally, practice loading on days when you aren't in a rush and don't have anywhere to go. That way, you'll have plenty of time and you won't have as much nervous energy for your horse to pick up on. Hope this helps! Good luck!