You can do it simultaneously, but it isn't like the Army where there's a special program for it. The cadets I know that did both essentially had to balance two different lives, missing AFROTC training for deployments, etc., whereas the Army won't deploy you as a dual-role cadet if you're in the simultaneous membership program. The Army also integrates your ROTC training into your role at your unit, where the Air Force doesn't.
My personal recommendation is to do one or the other, especially if the cross town school is far away from your son's campus. The cadets at my detachment who did cross-town and were in the Air Guard tended to have lower commanders rankings, not because they were bad cadets, but because they missed out on a lot of ROTC activities or were deployed for a whole semester and it was pretty hard to evaluate them as a result. This year for EA selections commander's ranking looked to be the make or break factor for a lot of cadets.
With that being said, I'd talk to a recruiter for info as well. The Guard and Reserve are an awesome opportunity especially for someone looking to fly, and many cadets do both AFROTC and the Guard at the same time and make it work. However, the two programs exist separately and don't integrate like the Army