I had a recurrence the very same year. It's well past two years for me now. That said, mine wasn't as severe as your father's, either time, but I think those quick recurrences might just mean that there was a tiny bit hiding out that was missed, and the second time is a good way to catch it and be done with it ... not a death sentence. See if that helps him a bit.
Now, my mother had a different opinion, because the recurrence was in the same spot as the original ... she thinks the surgeon missed something. But as all the reports were coming back 'clear margins' as he did the surgeries, I don't think he did. And it helped me to believe that, since I had to rely on him for my tongue surgeries and the neck dissection and some of my after-care. (He's a good doctor.) In fact, I'm still not sure WHY we even did the neck dissection, several months after the first surgery, because all the indications at the time were that we'd gotten it all. He was just that thorough ... wanted to hit all the bases.
Anyway ... nobody can know for sure, of course. It scares me that they seem to stop tracking folks after five years. I've hit that point, nearly, and what does that mean? That we're good to go? That we're not good to go? Or more likely, that nobody really knows. None of my doctors ever worried about giving numbers (not the good ones, anyway ... had one scary guy try to give numbers in the beginning, yikes!). You're right, that does seem a really weird way to word things. Is it possible to have the doctor clarify what he meant? He's the one in the best place to give reassurance, or at least backtrack if he misspoke. He's the 'authority' in this case, so the one best placed to give comfort, since he's the one who introduced the fear in the first place.
That's a bit rambly, but ... you're not alone (or your dad isn't, anyway) ... no matter what they say, it comes out scary. Maybe this doctor can rephrase things and sort it out better.