RE: Alpha 40 Trainer
I learned on my Alpha 40 last summer. I had very good luck with it, as do most folks who get them. Functionally, you really can't go wrong with any of the popular trainers on the market: LT-40, PT-40, Alpha, Avistar, Tower Trainer, Tiger Trainer, Goldberg Eagle, NexStar, etc... They are all designed to be really easy to learn on, and pretty durable for beginners. A lot of the decision making comes down to what you think looks the best, what kind of radio gear your instructor might have, budget, etc...
Don't let anybody convince you that more expensive=better. You can learn to fly just as well with a Tower Trainer as you can with a NexStar.
The biggest step is deciding to get into the hobby. After that, you really can't go wrong. And taking it up with your son will make the experience even better for both of you.
Good luck.