Dr. Weiss tone and response in the end (spoilers)

In the end when Birdy finally talks to Al, Dr. Weiss comes in and Al is like he talked! Birdy say something to him(or something to that effect) and I found Weiss's response somewhat odd, he says "Birdy?" in a way that implies either A) He's never heard him called that before or B) There is no Birdy and Al is imagining the whole thing. In fact at that moment I thought for sure the movie was going to have a twist ending where we find out Birdy isn't even there and its just been Al in the mental ward the whole time alone. Maybe the actors response was meant to be him asking Birdy to speak... but I dunno.