I just started my 4 yo Percheron stud under saddle this month and it was not nearly as hard as most of the younger horses I started. Within about 20 minutes of catching him from the pasture, I was riding. The next ride, we went to the country, and the next ride, we went through town and bareback. I don't think that all drafts are that easy though. Like solon said, when your groundwork is perfect, then give it a try. The biggest problem I ran into getting mine broke is that I can't get my 40 pound saddle lifted up that high. LOL. My dad has to bridle and saddle him for me because I am not tall enough. Since yours has been driven some, the transition to saddle should be fairly easy because he should already have the stop and turn principles down. When I take all my horses to the country, I kinda let them go at their own pace to find what is comfortable for them. The first time that we went out, he just proceeded to do a lovely flat canter across the pasture. He was not scared or running off and the instant that I signaled for him to slow down, he did. That was such a rush. My dad made fun of me by saying "It sounds like there is a steam engine coming across the pasture."
Oh, and I want to mention that if you free lunge him in a round pen with the saddle on, I advise you to tie your stirrups up. John managed to catch one of mine on a panel and rip it clean off my saddle. LOL He kept trotting as if nothing had happened. Not even a flinch when the panel jumped and the stirrup leather ripped. (western saddle)