I have heard this before..... I wonder if Mom's reaction isn't a defensive one to save her from guilt. Mom's are never supposed to be happy to send their son's (daughter's ?) off to war.
Nevertheless, many enlisted sign up and don't tell their parents. My sister-in-law joined the reserve's while a junior in high school 20 years ago. She was only 17 and got her Dad to sign. Afterwards, they both broke the news to her mother. Incidentally, she was a drill sergeant who spent a lot of her drill summers at Camp Buckner and Ft Lewis - taught cadets how to shoot!
My brother's step son dropped out of college after a semester - that winter (of 2001) he went to a recruiting station and joined the Army. 82nd Airborne division. He had not confided in either parent or his siblings and was 18 years old. His mother was shocked and not pleased. A badly sprained ankle kept him from boot camp until late fall of 2001. He went from boot camp to airborne school and the day after graduating from Airborne school he shipped out to Afghanistan. He also ended up in Fallujah in the early days of the war. For the 3 years he was Active Duty - he was in the US for about 6 months. He is home, in the reserves and back in college paid for by Uncle Sam and his mother is the first to tell anyone how proud she is of the man he has become.