It’s not the green card that is immediate but the relationship - parent to child = immediate where brother to brother doesn’t. The IR visa means Immediate Relative, not immediately issued a green card.
I don’t think he can do ROTC until he has some kind of legal status, beyond DACA.
Did you apply to any of the meets full needs schools like Berea or schools with a lot of FA like Alabama? If not, take a gap year and reapply.