Post 9-11 GI Bill & Montgomery GI Bill

My son (currently in Basic, PIR 2-3-12) had signed up with the Post 9-11 GI Bill when he signed up in May.  We received the paperwork that he sent home around week 4 and noticed that he signed up for the Montgomery GI Bill (MGIB) at some point in his first 7 days at Basic.

Question...is this in addition to the Post 9-11 or instead of it?

I called his recuiter and he said that he definitely has the Post 9-11...but what's the deal with the MGIB?  He couldn't tell me.  He said that it was paperwork that they did at basic and that he wasn't sure.

He signed up for a $100.00 deduction per month for a year and that will go toward the MGIB.  I looks like he will use the P911GIB first and then the MGIB after that?  Is this correct?

Please help me be less ignorant on this...I just don't want to see him sign away any benefits that are due him.

Jerry