He has always had a short stride and I will try poles
Hey! Poles help to improve the strides, put up to 4/5 poles! The distance depends on the horse, maybe start with smaller distances and as you pratice make them bigger!
Hope it helps
Maybe he is feeling stiff. Try and stretch him every time you finish riding. (Of course maybe getting the vet out if this is a change in his behavior- if he had a normal stride before)