Anytime from 18 months on is the time to break him to drive.
Draft horses are wonderful.
They retain what they are taught at an early age. You can turn a drafter out for a year or two and then hitch them again. Except for being a little awkward and out of condition, they will remember all of their early lessons in a very short time. Don't let your guy stand around till they are 4 or 5. Get an early start on their training before any bad habits are formed.