Do not delay in correcting this or it will work against him.
If you have a lawyer you trust and who won't rip you off, call ASAP. (S)he should just tell you to find you local CBP Office or even a POE if that is the closest. A simple letter of explanation from the lawyer (or you or dad) stating it was a simple mistake.
They will wonder how it happened so, I am guessing that there was a language barrier?
Normally, when one enters as a B, they are asked at least a couple of routine questions about the reason for the trip.
A greencardholder stating that they are just "on vacation" or "visiting" and entering on a B visa without another thought (rather than saying I'm on my way home or returning from vacation or a business trip, etc...) does not look too good.