Bought a "pure pitbull" but looks like a mastiff! Any opinions?

So I went on Craigslist looking for a pitbull puppy found a breeder who had one left. Went to check him out and immediately fell in love. I saw his dad who was a pitbull but his mom wasn't home for me to meet. As he's getting older it's clear he isn't a pure bred. From the research I have done it seems as though he is half pitbull and half mastiff. Anyone familiar with mastiffs? What do you think my dog could be?