Heyy there,

One of the best way to join Indian Army after 12th is to give the NDA entrance exam or go with the TES entry and for both the age limit is 16 1/2 to 19 1/2 and you will apply apply the application form only when your age is between 15 1/2 to 18 1/2. And your ward will cross the age of 19 at the time of his passing the 12th exam. So he will not be eligible to Join Indian Army as a officer.

Another method is that he can participate in rally of his stata where the age limit is 17 1/2 to 21, so here is that one chance where he can join Army after 12th as a soldier.

Good Luck :)