You are not getting a letter from Army ROTC guaranteeing any branch of service.
When you are commissioned you serve the needs of the Army at the pleasure of the commander in chief. That said - the branch into one commissions is assigned by merit.
Those who rank at the top get their first choice. Merit is determined mostly by college gpa, and performance at LDAC - Leader Development and Assessment Course which all Army ROTC cadets take at Ft Lewis the summer before their senior year.
Sometime in the fall of senior year cadets rank their branch preference - there are some programs in the Army that will allow a cadet to get their branch of choice for extra obligation.
I would encourage your son to go to Home
to see the wide variety of branch selection available in the Army.
Also keep in mind that 4 years is a long time in a young adult's life - long enough to have other learning experiences that will cause him to want to serve in another branch. I suggest going in with eyes wide open.
I encourage him as well, to talk with the Professor of Military Science about is desires and options available. Ask how many cadets get their first choice - this will indicate they were well prepared for LDAC and performed well there.