Early April (I think) 2017. The letter from the college needs to include acceptance into his major as well. My son had to wait to accept his scholarship because he switched from engineering to computer science - and the acceptance letter he had from the University was for the college of engineering. They sent him another which stated he was accepted into the college of sciences and arts - and he accepted the scholarship that day. You have until the end of May to accept the scholarship, but there really is no point in waiting, if he knows his school.
There is a handbook download on the AFROTC portal page - if he hasn't downloaded it yet, he should.