No one can really answer your question until ICE decides to do what they do.  It sounds like your dad picked up a felony DWI if he has been in for over a year and has been sent to a prison.  They will usually deport after a felony conviction.  Sometimes they will wait until he is paroled before they decide to deport.  Sometimes they decide to deport before a defendant paroles.  In the end, the decision is really based on the manpower and decision making process of those handling his immigation paperwork.  Regardless of which situation applies, he will mostly likely be deported.