If he has formal training in art therapy as a modality (as opposed to simply being open to clients sharing art in sessions), it will be part of his role to purposely remain neutral about the work you produce. This is in order to keep the focus on you and your responses/processes, as well as not praising you for producing "good" work. The quality of what you share is irrelevant after all, he is not a gallery curator.

I was in art therapy for a couple of years and it was immensely helpful. One of things I really appreciate when I look back is how the therapist kept the space "clean" of his input to allow me to explore what I needed. By contrast, my current therapist, who is not an art therapist, has emotional responses to art work I take to session. She is very moved by it, complimentary, wants to get involved with the work by holding it and so on. This can be thrilling and intimate, but it can also be double edged. When she doesn't respond like that, I feel as if my work isn't good enough.

What I am trying to say is that his neutrality might be an opportunity for you to dig deep into your self without concern about his self.