This sort of thing is why I prefer ramps! At our farm, we have a “bridge” used for trail classes. Something like that would be a good way to teach your baby how to step up and/or down (and I use the words so my horses will know what I mean once they have learned to associate). It is something like a pallet, only longer (a bit longer than the average horse) and made of much sturdier wood. If you or a friend can construct something like this, he can learn to step up or down out in the open where he can see, and hopefully transfer the lesson to a trailer, whether he needs to go forward or backward.
It is never the horse's fault