I heard, they draft the young men who are in good athletic. Both of my two sons have flat feet and younger one has curve back. I do not think that they would annoy them with letters or phone calls.
I am surprised that they annoyed your son. It seems not fair to burden on your son. That is government does their job not your son. I would change my telephone number if they consistently harassed my sons.
My son just turns 18 last December. I did remind him to fill out the form. He keeps postpone. Grrrrrr ! I will get his ass to fill that out form. If he doesn't do this, he could arrest or big fine on that.
Government is suck!! I would be freak out if they draft either of my son from my nest !