Don’t let a step up trailer discourage you, horses can do amazing things when taught with calmness and patients. Ask your yearling to put one foot,(one foot only!) on/in the trailer, stop, and ask him to back away/off using a cue word or signal or both. Do this exercise a few times or until he does easily without fear. Then ask him to put both his front feet on/in the trailer, stop, and ask him to back away/off using your cues. Do this until he does it easily without fear. You’re going to keep inching him farther and farther into the trailer asking him to back away/off without fear. Remember to stay calm and take things slow. Yearlings are still discovering that they have back legs and how to keep track of where they are, so try to have him only put one back leg onto the trailer before asking him to back away. He will want to put both back legs on by hopping up, so you will have to read his body and try for one back foot on the trailer. By having only one back foot on and one off he will realize/learn where his back feet are and the height of the step. Repeat until he is calm and without fear. Then ask for all feet in, repeat.
Remember to use your cues, stay calm, be patient. Use lots of praise and stroking. Yearlings do have a short attention span so even though you are working slowly watch how much time you are using. Quit on a high note.