Laurapratt, he is beautiful!! Lower those shaft loops a hole or two so that the cart seat levels out, and you will be good to go!!
You may find you want a set of pads for the harness, as the breast collar looks a bit fine(like a show harness) and they will be more comfortable on trails for him. The breast collar could possible be raised a hole, too. It should be above the point of his shoulder, but not hitting his windpipe at the base of his neck.
Have fun driving, and NEVER take the bridle and reins off before you unhitch, and never hitch before you put the bridle and reins on!!