I own both DB and DBPro and I can say both are excellent for what they do. If your son wants to learn, or program at a hobbyist level, DBPro will be both fun AND a great learning experience. Like all systems it has its flaws though and getting past it's bottlenecks and creating a commercial product is possible, but very challenging.
Man I'll never forget the first time I stayed up all night experimenting with DB. I made a 3D boat on moving waves that you could drive around. It was inspiring!