Schools are clambering to get 3 year scholarships, especially if you son will be are part of the Corps of Cadets at A&M. Its far less money the school has to shell out in scholarships/grants later on. I can name quite a few schools that compete for kids with 3 years, by offering significant scholarships like paid tuition for their freshman year. We speak from experience, so talk to your schools Financial Aid team, and inform them that your son has a 3 year in hand.
It will greatly help having a competing second school or even a third school already accepted into that is offering something for Freshmen wit a 3 year. In our case, both military public schools our DS son was excepted to were offering tuition freshmen year then room and board soph - senior year. We took that to his third school and they trumped both schools, and was the right choice for him.
Its just business and negotiations. Schools want them, maybe even more, then they want to go.