I love to practice 3 loop serpentines but using only leg aids ( or very little rein) to help the horse understand to yield to the the pressure! Once the horse starts to understand it, I begin to yield to each letter instead of doing a half circle.
circles, circles, circles!! I have a very stiff horse and an amazing trainer/mentor taught me to take my inside rein (let go of the outside rein) and lock it behind my knee and make the horse walk in a small circle, the minute he yields, unlocks his jaw, and moves around my leg, I release him into a straight line. Do it EVERY day as your warm-up, from there you trot the small circles, then eventually canter- the horse will learn to put slack in the inside rein by himself! It can take anywhere from 10 circles to 10 minutes for the horse to yield, but just be patient and the minute you feel him give, let go. As Richard Watson told me, its all about feel! good luck!
In hand work is also wonderful for improving your horses response to you aids.
I do leg yielding away from the fence, in trot and canter, so he does not fall out taking the easy route, spirals moving his quarters in and out to get the right, in your case left, hind stepping under more, lots of counter canter, it is good for straightness and makes the rider sit evenly as well as engaging the back end.
Pole work is useful as they really have to work evenly, use a long line of poles or poles on a circle, again good for the rider as well as the horse, you can get them working correctly without making an issue of it which can in itself become part of the cause, sometimes the harder you try to correct something that they find difficult it can cause more tension which then makes the issue worse.
A lot of stretching and relaxation! Work hard and I'll gain on muscles and it will be easier for him to yield :)