JMHO,
but this I don't know is it's own answer.
Of course he knows, he just isn't sharing/ communicating, putting into words his current feelings/thoughts/reasons, etc.
Again just my opinion, but this "I don't Know'" is a nice blow off tactic, as in I'm not dealing/coping with this today.
Somebody would only have to tell me I don't know a couple of times, and i am certain I would know for myself very soon after that.