Janet, there is a lot of training that can be done before pulling. He can learn all the drafting commands now. You can get the DVDs and book from Wilczek Woodworks and do all the prelim work, getting him used to the harness, a sacrificial tree, a fake pvc shaft frame, pulling empty plastic containers, getting directional commands from beside, behind, in front, learning to back up etc.
You can set up obstacle courses and just walk him thru so he gets used to the idea.
I used a Wiggles Wags and Whiskers harness to train for perfect loose leash walking and drafting / joring commands. It's been very helpful. Before I actually hitched any weight to Orka we took him to the vet's for a thorough exam, explained what we wanted Orka to do. He had had X-rays a few months earlier at an emergency clinic (nothing wrong but an intestinal bug, ruling out obstruction) and the vet looked them over, plus the hands-on exam, said his front growth plates had already fused and that he was in excellent shape and cleared for all the sports I want to do with him.
Of course I've also been exercising and conditioning him steadily for 1 1/2 years. Just started caninordicwalking with him -- he's pulling me walking with trekking poles and with a skijoring X-Back Harness, bungee canicross towline, skijoring belt -- and he loves it! We're both exhausted and sore though, sort of a surprise but all the books warn it uses different muscle groups so we skipped hitching up today. Normal walked 5 miles this evening plus he played at a dog park and frolicked in a river.
Because of all the pre-training and pre-conditioning I was able to hitch him up and he "got it" within 5 minutes, just an awesome fun feeling, like flying compared to regular walking, caninordicwalking. He'll be pulling a cart (no weights in it) soon. He's 20 months old and robustly muscular.
I thought it would work out well because he loves to run, to pull, to fly ahead on trails, and drafting is not far from all the walking and hiking we've done; it's natural. Oh, and he's watched the DVDs with me of the Bernese Mt Dogs, and he watched a Newfie Draft Trial with me.