Hello, and welcome, you have come to a great forum
You may get different opinions, but in my humble opinion, a horse's stride is based on his conformation and he will move on the way he is built. Or, if he had an injury at some time that affects him, but that is probably not your case. You can make sure that he has plenty of exercise to keep his muscles toned so that he is fit, , but the length of his stride will most likely stay the same because his genetics has formed him that way. Personally, I would not use anything artificial to try and change him. Are you thinking of driving him? I have seen many short striders in the cart and they go just fine.