RE: Alpha 40 Trainer
As you've seen in the replies, everyone has a favorite trainer. Often it is what they learned on. If you look at RCKen's "sticky" at the top of the 'Beginners' forum you will see that there are 50 or more ARF trainers that are proven good models. The Alpha is a good choice, especially if you and your son prefer the color scheme and graphics of it over others you have seen. With so many good choices, you might as well pick one that looks good to you and your son.
Totally unrelated to the original question: All kids are different so this may not apply. Even after your son becomes a good pilot, you might want to keep him on the trainer box. Really young kids often have a short attention span and do not always realize the plane needs constant attention. A few years ago a guy told me about flying with his young daughter. She was a goodenough pilot to fly solo, and was flying intermediate planes. In the middle of a flight, she stopped watching the plane because a butterfly few in front of her and she decided to watch it rather than fly the plane. Another time she just put down the transmitter, with the plane in the air, and went to play. Luckily he had the master transmitter so nothing bad happened. He said he never had to take control unless she decided she was done with flying and started doing something else.