I would just use the tower trainer to teach your son on. If you have a computer, a flight simulator is a big help. It will teach you how to use the controls and keep you from getting disoriented, especially when the plane is coming towards you. Plus he can crash as much as he wants and it won't cost anything. If you haven't already done so, join the AMA and find a club in your area. If you don't feel comfortable doing the training, most clubs have instructors that can teach you to fly.
I've never owned a glider so I don't know if they are a good teaching tool or not.