has been at boot camp for the past nine weeks. approximate. >> yes, drill sergeant. >> reporter: from the beginning we followed him through his training, chronicling his toughest challenges. >> get off the ground! >> let's go! >> are you seriously coughing and crap? >> reporter: how would you explain to a apparent wparent w have a son or daughter going through this what it's been like? >> you couldn't explain it. >> if you don't have one, you have no clue how you would treat it or what you would do. >> reporter: his parents say they see the impact military service is having on will. there's the physical transformation. they saw it bh will was home on holiday leave, but it's his he letters, they say, where they see the most striking change in him. >> william's never been a person to pour his emotions out. he's always held them in and