I would act as if the pony were NOT trained to drive, and Start it like we start the new horses.
Try dragging stuff behind him while leading. Clip a length of chain to a lead rope, tie that to the stirrups, and drag it on progressively noisy surfaces...start on dirt, then gravel, etc. then use some pvc pipes for shafts. If you are still getting almost NO reaction, then add some weight to the drags, like atv tires. At this time, though, you will need some type of breast collar to help with the pulling.
If there is ANY negative reaction along the way, I would assume he is NOT trained to drive, and NOT hook up a cart! If you have a driving trainer in the area, I would take him over and pay for a lesson, and let them do all this stuff with the proper facilities.