Did you get his boot pics? Ok...that was a really STUPID question...of course you did! We moms wouldnt be caught dead without those handsome pics of our boys.
I would do something like this:
Boot picture.
Private difficult child, graduate of XYZ High School, has graduated MCRD San Diego.
He plans on serving our country as XXX (MOS). As his parents, please join us in welcoming him to the United States Marine Corps! Semper Fi difficult child.