I think that all horses would benefit from some cross-training. If you want to try training, I would suggest that you find a good reining trainer that is willing to help you and him. I don't know if it would be counterproductive to his dressage training but I can tell you that when my Dad was showing all the time, he would do roping, reining, and WP (all 3) on most of the horses he showed. If he had one that had some extra cow savvy, he would also do a working cow horse class sometimes in addition to the other 3. That kept his WP horses moving out and natural, his reining horses balanced, and his roping horses calm.