He definatley is a Pit mix. I want to say with, maybe a black tri Australian Shepherd? With the blue eye, the way the ears sit, and the markings with the front white socks, white collar, and the blaze on the face? Just a guess. I found this picture on the internet, that said it was a pit/aussie mix. Your pic kind of reminded me of that mix.
I see the Boston Terrier markings sticking out like a sore thumb, but the rest is up for grabs IMHO.How much does he weigh? At this age, that is a good general indicator of adult weight, in my experience. I also get a feeling of Border Collie.