Most horses that have dressage training and compete in the upper levels have the potential and the muscle to jump. Smartness isn't an issue either. I'd definitely start looking into a trainer or you could train him yourself. It's not too difficult. Start by small things and don't ever rush him. Trot and canter over ground poles until you are both so comfortable that he doesn't even look at them. After that, put up the lowest crossrail possible and walk, trot, and canter over it. Let him sniff it, bite it, whatevever. The thing is, to take baby steps and never rush. And to just keep progressing forward.