21B is the Combat Engineer MOS. It will depend on what unit he is attached to, and if he wants to go National Guard, or active Army. Active duty, you have alot better chance of getting into the shit right away.
National guard, you can join almost any unit in the 16th Engineer Brigade, then voulnteer to backfill into a deploying unit, and they sill snatch you up.
Sapper school is a 2 month supper 21B school. 1 month of classroom instruction, followed by 1 month of living in the woods. I wanted to go, but no longer met the requirements to attend. Damn knee.
21B is usefull, you will probably get cross trained on some of the heavy equipment. After a while, it is easy to get into the 21E MOS as an equipment operator. Upon getting out, apply with ODOT or any larger construction company, any start making overtime like mad all summer, 3 month vacation in the winter. Some construction unions count military time as time in the union, so you dont start out as low man on the union ladder.