Get thee onto a 20m square. 1 side shoulder in, 1 side travers, 2 sides shoulder in, 2 sides travers. Once you (think) you have it, exaggerate the travers to a really open 4 tracks. Rinse repeat until you both get that you’re just changing his body position and bend and it becomes easy. Don’t fart around with your weight. Make your leg and rein aids as small as possible with the goal to make them smaller and smaller.
Once that is on the way to beautiful and it’s feeling easy hit the long sides with shoulder in to renvers to shoulder in. Make sure the shoulder in is an exaggerated 4 tracks so the renvers is also exaggerated 4 tracks.
Make some full passes in walk - allow transitions to trot if they happen (but not to canter obviously).
Finally, go onto the long diagonal and make a shoulder in, gradually turn his body to travers on the diagonal, get a few steps and go back to shoulder in, finish the diagonal and repeat the other way.
This might take a week. It might take months. It will give you a very solid trot half pass.
Oh, also, throw in some increasing and decreasing the gait in shoulder in on the long and short sides. By some I mean a few strides - spend the entire long side speeding up and slowing down/lengthening and collecting. This will instill the possibility of adjustment within a known movement, during collection, while bent.
Note, the exaggerated angle does two things. It first pushes past collection allowing freer movement and second, it allows both of you to realize that sideways is fun “Whee look at us going all sideways!” because it takes some of the difficulty away but fully allows the realization that bend and sideways are possible.
Finally, spend time feeling your own seat during every warm up. If you have an indoor and your horse is safe enough, spend good sections of long sides with your eyes closed feeling how your pelvis moves in walk, trot and canter. During these times, allow a following seat only - don’t influence your horse, just feel. Be sure to feel the lateral pelvic movements as well as the longitudinal movements.