Common Courtesy

So I volunteer for a family for about two hours each week (or more) in helping their daughter who has autism. I have known this family for some time now and I willingly do this since I want to become an OT and focus on children with autism. Today was a fabulous day while I was working on tasks with the child, we were drawing shapes and she drew a smile face and dubbed it a monkey. Then we drew a circle she then added 2 eyes and a mouth and something on top of her head, she then looked at me and said "a queen." This took me by surprise because her imagination was working and we have never had her do this task before. Her mind is developing and her imagination is growing and it was really cool to see her be able to come up with this imagination. We scribbled for a bit and looked up and said "the ocean" and then proceeded to draw a king. It took my breath away to see her being able to react like and watching her grow each week just amazes me. However, my day was ruined when I went to leave. And when I mean ruined I mean that I was let down. I left saying good bye to the autistic girl and her sister I then proceeded just to say good-bye and the father completely ignored me. This is not the first time he has done this and I feel that he purposely ignores me because he knew I was leaving it wasn't like I was with a group or sneaking out, I was the only one leaving. He does this often and I normally let if roll off my back but some how I can't do that this time, it really is bugging me. I know that I have not done or said anything that is rude because I normally do not hang around the house long enough but I just don't know what I should do. Should I tell him I think what he does by ignoring me is rude or should I just let it roll off my back. The way he treats me is really getting in the way of my relationship with the girls because when I know that he is going to be at the house, I do not want to go over for a visit.