Enlisting into the Marine Corps can be quite the transformative experience. For some, moreso than others. While many are able to resist the temptations of becoming a total toolbag upon graduation from boot camp, there are just as many who succumb to the irreversible identity changes boot camp instills in young Marines.
What was once a chill, regular dude you went to high school will return home with a fresh high-and-tight haircut and a moto shirt he bought at the MCRD PX on graduation day. Instead of speaking like a regular person in your town, he speaks in strange idioms and manners he picked up from his Drill Instructors. He asks you, repeatedly, if you “undastand” him, or if you’re “tracking.”
Be patient. Do not blame him, for he knows not what he has become.
Give him a couple of years in the fleet to wash off all the moto, and slowly return to the person he once was. These things come in cycles, but be assured he does still exist in there somewhere.