So as someone who teaches robotics to middle school Students, FIRST lego is still the best program for kids. If you would like to learn CAD, EE, 3D printing, and microcontrollers, then certainly go the battle bots route. However, this will be a steep learning curve for your son and he may get frustrated. With FIRST, he will move from FLL to FRC in late middle school which has much more open road. He will also learn to work as a part of a team and solve problems in concert with peers which is different than a father son team. FLL is intense and almost a year round pursuit. Find a good home school team for him to join and he’ll get the best experience.