Sig LT-40 Kadet with O.S. or Thunder Tiger .46 engine
Tower Hobbies Trainer 40 or 60 Note: IMO Stay away from the Tower Hobbies Engines
Hangar 9 Alpha Trainer 40 or 60
Lanier Explorer 40
Any of the Hobbico 40 size Trainers

Any of these Trainer Planes would be fine to learn on. For a Ready To Fly Plane, the Alpha 40 is a pretty good bargain and a good flying Plane. If you buy an Almost Readt to Fly Trainer and buy your own engine and radio seperate, the Tower Trainer is an excellent bargain. The Sig LT-40 Kadet is probally the most gentlest Trainer in the bunch, but slightly more money than some of the others. Basically a Trainer is a Trainer as long as you buy one from a reputable manufacturer like Tower Hobbies/Hobbico, Great Planes, Sig, Lanier, Hangar 9. My advice would be to stay clear of some of the no name brands, and Vmar. You may want to find an instructor first and see what he or she recomends. Same goes for radio system. An instructo may prefer you buy a certain brand Radio to match up with his or radio while using a buddy cord. Also remeber that a Flat Bottom Wing Trainer is more docile than a Trainer with a semi symetrical wing. The symetrical wing Trainers(Hobbico Avistar, Hangar 9 Arrow) are a bit more aerobatic, but still good for learning on as long as an instructor is used. I also suggest that you and your son join the Academy of Model Aeronautics.