Hmmm, well it is his only daughter.
If the only thing in your world that you have left to love changes into something, but you cant know the full truth or you will lose her. What would you do...
Jacob mentally prepared him sorta, So he would have assumed she was akin to him maybe... We were also drilled that he was a man of few words, so why would he react now. It is simply have your daughter in your life and know very little or be told she is dead and suffer or have her leave and never see her again.
We could assume he was in shock of sorts... So he didnt know what to say, since we know he is not a eloquent fellow
Sorry if this doesnt make sense, im watching a show right now, if people dont understamd I will edit.