if he has been driven he will probably be used to the left rein being kept steady, to keep him from falling in, whilst you give with the right rein, to turn left, and vice versa for right turn. and sawing on the mouth is generally not good. nor is reinflapping. i would advise some long reining using voice commands, with general groundwork and when he is responding well to you, i would attatch a log or small car wheel behind him with some baler twine which you can easily cut if you need to get him out of it in a hurry. get someone at his head while you try him out first too. and if he will pull the tyre or log happily for a week or so, i would then think about putting a cart behind him. even if he has been driven in the past, if he has had a gap, then he will need a refresher course, so to speak, and it is far easier to sort out any problems in easy stages, rather than end up with a big expensive or dangerous problem because of rushing things. ie. no point in getting hurt or wrecking a perfectly good cart.