Then your son wouldn't be going to that school.
Have you asked the military school if there is another option if the current school won't provide a letter because he hasn't been there long enough?
So are you willing to let your kid get booted from the mil-school when they find out the recommendation letter was forged?
This thread has moved beyond any legal issue. If the military school is insisting on a letter that you can't legally obtain, then you'll have to make other plans for your son's further education.
You would be setting a terrible example for your son.