How to slow down a pony's trot.?

I have gotten into a business deal with a lady I've known for years. We are selling a 14 hh Trakhener/Welsh cross. I had ridden him for her a few years ago, so I am familiar with him. He is very talented to be such an underdog. He's gone through training level eventing and he is really talented in dressage. However, there are two problems with our flatwork. 1) Since he is a pony, he does have very short, choppy strides when he trots. How can I help this? I have tried pushing him forward with my leg and seat, but he just speeds up. I need him to at least slow down his steps. Ideally, we could lengthen them as well. 2) Our canter is fabulous, but unfortunately our canter transitions are not. I can't seem to set him up well for neither the up or down canter transition, but I don't usually have problems doing that with any other horse. He's just very different from the horses I am used to riding.