The 3 year AD scholarship itself does not kick in/ start until one completes their first year at the university, demonstrates they are meeting standards/ grades/ PT standards and then the scholarship and benefits start as a 3 year scholarship on day 1 of year 2. However, what the posts above are noting is there is some opportunity, though not a guarantee, that those knocking the cover off the ball may have their 3 year scholarship upgraded by their unit command recommendation to a 3.5 year scholarship to cover Year 1 Spring Semester costs. Overall your son should focus on excelling in his grades, learning like a sponge with a good attitude in his ROTC program, and optimizing his performance in PT - I will say it's nice once they reach a certain fitness level added PT or PT evals are low stress - they are ready.
As for contracting early, your DS should work with his academic and unit advisors on his academic plan- if he has a path that the leadership approves, he may be able to graduate a semester early. One of my children started college with a slew of AP/ dual credit courses and is taking 20 credits a semester to graduate a full year early - she could do it in 5 semesters but will take six in order to continue to compete (rock climber). Just look ahead to ensure graduate programs would not like to see those AP 5s taken at a college level - like top med schools want to see for all science/ and some math courses, generally.
There is a sequence, pyramidal in how the Army develops officers, and a part of that sequence includes progressive responsibility in 8 semesters and summer field training. If my DD or DS were going Army I would encourage them to take all 4 years in order to fully prepare to lead soldiers once they commission.
Good luck and hope that helps.