Your best bet on scholarship stacking info is to call the bursar's office. I'm sure stackability will vary with each scholarship. There will be a few opportunities to apply of a scholarship freshman and sophomore years. The first will be to (re)apply for the high school scholarship. I know they have a 30 credit hour limit but I'm also pretty sure that if your DS will be spending 4 years there then he will be able to apply. In any case his MOI will be able to help him through the process. BTW - do you know if Auburn will accept all his previous credit hours? Might required courses for his major keep him there in any case?
DS did not win a scholarship out of high school. Through hard work he was awarded a scholarship the middle of sophomore year. I think it made it so much sweeter that way. IMO the only thing better than being a MO midshipman is being a Marine officer. Good luck to your DS. Good times ahead for him. Tell him to give it 110%.