If he’s willing to eat in the trailer, he’s pretty chilled out. I think this is just young horse stuff.
Not going into a trailer (whether they like it or not, whether it’s bumpy or not, whether is the biggest rattling can of iron on the road or not) comes down to not yielding to poll pressure. This needs to be a non-negotiable item in his life.
When a horse is reluctant to move forward towards the trailer, I personally do not tend towards the “fire and ice” approach with repetitive lunging outside the trailer - I find this just upsets most horses more, and that’s the last thing you need at that moment. I just stand in the trailer and become THE most annoying person on earth. TUG TUG TUG TUG TUG TUG TUG on the leadrope, not a steady pressure but rapid and pretty firm pulling. Any iota of effort towards getting in gets a 10 second “think about it” time where literally nothing happens - no pressure, no clucking, I don’t even look at them honestly. Count the seconds out, it’s longer than you think. “Iota of effort” includes even THINKING about getting on, or stepping forward. You have to reward every single tiny effort towards the trailer.
Then, slowly but not too slowly, I turn into the most annoying person on earth again. TUG TUG TUG. If the horse shoots backwards, I just go with them, continuing the tug-tug-tug until they take a step forward and then IMMEDIATELY the pressure disappears.
I have yet to find a horse this method didn’t work on. Timing is key. No escalating beyond what response the tugging gets.
The key is when they FINALLY get on the trailer - tell them how great they are, and then back them back off again to do it again. It’s not a once and done, buddy. The same way you’d target 3 good transitions for one bad one during a ride, same thing goes for trailer loading. If he hesitates, he just earned himself multiple trailer loading practice runs.
I wouldn’t over think this too much. This horse is young, and you need to do what every young horse needs to do - practice.