If it were me I think I would recommend staying with the scholarship in hand. That way, just in case something happens or minds change the option of active duty is still there. He can always choose guard when he accesses his MSIV year.
The service obligation is the same. With National Guard it starts after you report to your guard unit, the SMP time does not count. With active duty it starts when you go to BOLC which could be an unspecified time after graduation.
DS is an MSIV this year so btwn his ROTC commitments, his guard drill once a month, and his academics, his weeks are pretty full and does not leave much time for anything.
Also, if his school is not in his home state, the guard unit he drills with may not be as receptive to an SMP cadet they know is going to leave when he graduates. To some I'm sure it doesn't matter. But just something else to consider.
Best of luck to your son.