Some would travel better in a lorry after a trumatic incident in a trailer, and vice versa, but what you could try is (if you have a front unload), is starting with no partition and just walk in and walk out a few times, then brace the partition a quarter to a third of the way across, so the walk in gap is smaller but still large enough, do this for as long as it takes until you feel happy to stand him in the box with a feed, and then start the process again until (hopefully) you have the partion in the right place, and he is happy and settled either with a feed or haynet.
At this stage and you have a smooth enough field/drive take him on a 1 minute journey, bring him out, make a fuss turn him away.
Then add the other horse again at a standstill, so will need a willing horse who will just stand happily munching on a net. The gap is how it should be and he will walk on and off, without rushing, then pause him in the postion where he would normally be tied up before moving him off again, and repeat this until he is happy to be tied and go on the 1 minute + journeys.
Hope this helps