I would say, unless he wants to be a nuke officer, apply for STA-21. At worst, he doesn't get selected, at best, he does. When he's eligible for LDO, if he's still interested, apply for that as well.
As a side note, either way, he should start taking college courses now (if he hasn't already). If he gets a degree before being accepted to STA-21, then he can apply for OCS.