I am going to ask if you have a 2-wheel cart to drive him in? This is much safer to start with than a sleigh! Cart follows his motion, forward, backing, easy to turn.
While sleighs of the cutter type are light, you CANNOT back them if horse gets dancing in front of you! Horse has to learn to push shafts for turns. He may have gotten the basics of pushing in ground work dragging poles. Wheeled cart makes pushing shafts easy for turns. Does he understand stepping sideways, pushing into resistant shaft, WITHOUT also moving forward? He needs to learn this, you will use it often, especially driving sleighs.
As explained to us, sleighs run on a layer of water caused by friction of the runner on snow. When you stop moving, the water layer refreezes FAST. So sleigh horse is always stepped one to the right, two steps to the left, to break the frozen runners free. Then asked for forward motion. This needs doing EVERY time you stop the sleigh! Even very experienced carriage driving horses can get startled when sleigh doesn’t move easily or at all, with frozen runners. I have seen horses rear, spook, kick, thinking SOMETHING had a hold of them!!
That stepping sideways and pushing shafts weight is a LOT to ask of a beginner horse, unfamiliar with ANY vehicle behind him!
DMK gave good advice about dragging things to get him fit, used to load resistance behind him. We drag tires with our young horses. Bigger horses, so a dually pickup tire is our choice. You are looking at resistance against the dirt, not a big, heavy load to move. He gains confidence he can ALWAYS move his load, starting while on the long lines. Funny how you have to teach them that confidence, starting small and building weight over time. You never allow him to fail by overloading him. They truly do notice weight changes! Flat side of tire smoothes the ground, maybe swings out on circle to change pull feel of traces on his haunches or shoulder. All good things! Tire can clear off-flatten snow depth in the ring to drive on with the cart. Our front barnyard is always really nice when training the young horse with the tire!
Do everything using blinders, that he already knows in the open bridle. Do not allow horse to “help” you by starting before asked, try to vary any routine in working so he is not anticipating the next step. Blinders are not to prevent seeing scary things behind him! Blinders prevent horse from reading your body language, seeing whip movement for anticipating or doing things you don’t want done…YET.
Whoa means instant stop, this is your “emergency brakes!” We use a prepatory word for slowing, change gait down trot to walk, walk to halt. That prep word is HUP. So I would say HUP, WALK, to slow from a trot. HUP WHOA to stop. For a faster stop it is HUP, HUP, WHOA. Horse should be stopping on the second step. He learned this doing ground work, hearing and rein signaled EVERY time. Horse WILL help you when you give him the preparation words! This is where routine use is your friend. HUP is a gait change, not a slower trot or walk word, we use EASY for that. Use any words you like, just use them consistently the same way. Horses responding as asked to words, has saved our hash more than once in situations!!