That's how my aunts horse was. He would protest everything. When she would ride him he would buck until she was off of him. She almost broke her ribs from falling so hard once. She realized that she needed to make him a gelding. After that he was good and none of that stuff happened ever again. Now idk if you're horse is already gelded but maybe try that.
With my mare a lot of things were connected to trust and respect. She didn't want to get into the trailer because she didn't care or sometimes the surroundings made it scary and she didn't fully trust me. Once I gained her respect it worked out fairly well. For some reason it worked when I didn't look back at her. I walked around with lead rope in hand and walked into the trailer not looking back at her to see if she was. like I was expecting her to do this thing and I wasn't asking so she should do it. Even if she resisted for a few seconds I would stand my place and just wait for her to calm down while still looking forward. Patience is really important and just try not to get her worried by getting angry or excessively pulling. Then again, all horses are different. this worked for my mare and not my gelding. I wish you luck thou and hope everything turns out ok :)
It's just like a kid, he's asserting his independence. I found #naturalhorsemanship method that works really well for this. I tried to find a YouTube video but it was taking to long, so you might want to try that off this description. Get a friend to help. One leading the horse, one behind holding a lead rope. Person at the head does not pull the horse on, simply keep the horses head straight. Apply some pressure but always, always release when the horse offers to come forward. Not even it's feet just, giving to the pressure you are asserting (this is a timing and feel thing so hopefully the person at the head has both). The person behind with the lead rope stands slightly behind and to the side and spins the lead rope. Not hitting the horse, but creating a moving object it wants to move away from (this is where I was hoping to have a video). This takes time at first, so practice not when you are rushing trying to get somewhere. Focus on your own jobs. Head person keeping horse pointed in trailer, tail person, keeping hind end square to trailer buy moving to whichever side needed to keep horse straight. Lead rope allows you to be far enough away so you don't get kicked. Hope you can find video. I trained horses and never had one I couldn't get on the trailer within 5-10 minutes with this method.