Start by just having them walk up to it, get them used to the idea of the drop, then try to walk them down the drop and if that goes well then try it at the Trot and the canter, you don’t want to rush a horse if it doesn’t want to do i
I agree with Lowri, start with very small drops and a brave horse to lead. Gradually take more space inbetween the lead horse and you, untill he jumps them alone. Then move on to a slightly bigger drop and start close to the lead horse again. Always reward him immensely for jumping down the bank, and quit in time. This way he will learn the drop is a fun exercise. Good luck!