Aerospace, Electrical, Mechanical, and Nuclear Engineering are all great fields to get a degree in if he wants to be an astronaut. Keep in mind though, that less than 100 people in the history of the world have been an astronaut. That's a .0000000001% chance. It's a great dream, but be sure to have a back up plan. Either way you (He) would probably land a job in NASA with a degree in any of the afore stated if he graduates from CalTech or MIT.