I gave you some incorrect information.
The skill level numbers I gave you were the numbers the Air Force uses.
The USMC uses different numbers:
“0” is used to identify personnel undergoing training for award of a primary MOS (PMOS).
“1” identifies a private (PVT) through specialist (SPC/pay grade E-4) or corporal (CPL/also pay grade E-4)
“2” identifies a sergeant (SGT/pay grade E-5)
“3” identifies a staff sergeant (SSG/pay grade E-6)
“4” identifies a sergeant first class (SFC/pay grade E-7)
“5” identifies a master sergeant (MSG) or first sergeant (1SG) (see E-8, below)
“6” identifies a sergeant major (SGM) or command sergeant major (CSM) (as of 1 April 2011)
Your son would be a 0 after Boot Camp, he would be a 1 after tech school (called infantry school in the USMC), and a 2 after a few years in most likely.