I would definitely try to presuade him to give you lessons...even if you have to trailer over there or he has to find time to drive out(which may or may not be possible).
There are many, many things that can go wrong with driving--especially as runaways and wrecks are so much worse because of the cart attached. That said, with a well-trained horse, it is a lot of fun and very relaxing.
Ask him if the horse ground drives well. If so, that may be an option to consider for a while. That way, if the horse spooks or something goes wrong and he runs, you don't have the cart to worry about and the horse is more likely to be 'unscarred'(mentally and physically) by the experience. That way you can make mistakes and they won't turn into big problems. If the horse does grounddrive, ask him to show you how the horse does it, before you finish hitching and actually drive. (Ground drive means that you have all the harness on, but you don't attach the cart. You walk behind them(or to the side, your preference) and 'drive' them.)
Make sure you know how to adjust the harness correctly and safely. If it is not correct, you are even more likely to wreck or spook the horse. Also, ask him to show you how the horse is harnessed--what goes on first, what he tightens, etc. Does the horse have any problems with certain movements/parts of the harness, etc? Does he tie well while being harnessed or is this a 'free-standing' trained horse(the horse isn't tied while hitching...some people prefer this).
Always have a whip with you. If you get into trouble where he needs to go forward NOW, a whip is much better than no whip. As you get more experienced, the whip is used to tap on the sides to keep him straight, move the horse over, etc. Ask if the horse is used to the whip signals, and ask him to show you.
Go slow and calm. Remember, the best thing is usually to stop and comfort the horse if he gets spooky--he should know that instead of running, he should slow down and wait for your direction(very much a trust issue). You can always unhitch, so if you feel nervous, unhitch as soon as possible. Make sure the horse knows what Whoa means and responds to it in all situations.
I wouldn't say driving is hard, necessarily...just a lot of things to remember--it becomes second nature once you have driven a lot. There is a lot more I could cover, but I will stop there. Good luck!! Hopefully the horse is one of those that take care of a newbie driver--they are absolute gems.