I don't see any reason there should be a problem w/ teaching him to drive. As you train him in ground driving, you will build him up so that by the time you have him ready to hitch - he's plenty strong enough to start going on conditioning drives.
I've spent the last two years training a pair of mares to drive - they are currenlty 20 & 21 years of age. They are awesome - at home, out at shows/presentations & trail driving as singles, pairs and as a 3 abreast w/ a younger mare. The one mare hadn't even been "properly" halter trained - could be caught and haltered but didn't know how to lead or stand or back up...