Lounging before riding, going out on trail rides, using serpentines and figure eights. I have ridden a very stiff horse, when I would ask him to trot the would immediately throw his head in the air and avoid the bit. I used a lot of transitions and would spread my hands out almost like I was riding a bicycle and would give the reins to him as soon he dropped his head while still maintaining contact. I would first do it at the walk, and then at the trot. While doing this consistently in his warm up he didn't avoid the bit and had a more supple way of moving. Not every horse is the same, but I hope that was helpful.
I use shoulder in, leg yields and spiral circles when my TB is stiff or resistant.
You can hire a masseuse to loosen the muscles and make him less stiff. Usually isn't too expensive. Ours charges $60 for an hour and it works very well
Your horse will be easier to bend and more supple once he has stretched his topline and is loose through his back. I would recommend really working on stretching him and then, begin to ask for a bend spiralling in and out of a circle, serpentines... Begin at the walk and take your time, don't expect too much from the beginning but make sure your horse uses himself properly. Also, lateral work is good to loosen up a horse and to improve his response to your leg, which you definitely need to get him to bend! Hope that helps😊