What a sweet baby! He is soooooo cute, mixed or not. If I were to take a guess, just based on those pictures....I would have to say that he is mixed. However, it's so hard to really tell with those pictures. My first thought was a Min Pin. I had a neighbor that used to raise this breed and he reminds me a lot of them. I know that you pointed out that you don't think so because your puppy is small....the Min Pins are a small breed as well. Whatever your decision, he is an absolute doll and it sounds like you are already in love with him.
Tammy