Hello Isabel,
I recommend you to work on the notion of comfort with your horse, to let him know that walking is more comfortable for you but also for him.
You can begin to free walk on loose rein and when he begins to trot you can ask him some exercises, halt, shoulder in, counter shoulder in, reinback when your horse calms down, reward him directly. Repeat the exercise as many time it is necesary. The goal is for him to understand that when he is calm he won't be distubed and that he will have to work if he doesn't want to stay calm.
The answer is mainly in the idea of rewarding him as soon as he calm down :)