Well to start with your first question. There are lots of things you can do to help him build up his muscles. Trotting is the best thing for him. If you have a round pen, do around 30 minuets of walk, and trot work for the next few days. Then slowly added more trotting time. Like 1-2 minuets every few days. Trotting is better balance for the horse while still allowing them build of muscles.
It vary much depends on the horse. You need to watch him each days. Just stick to the sled for a few weeks. You don't want to make him super sore or he may not want to be put back into the cart. After a few weeks add around 30-50 pounds, Or even a person. You just need to watch him, just don't go to fast.