Do you think my pit bull is mixed?

I have a 1 year old pit bull but i think he may be mixed. He weighs 105 pounds and is 26 inches tall. He is huge. He has loose skin like a mastiff.I was told that if there were spots on the roof of his mouth that means he is mixed,and he does have spots.Is that true?Either way I love him so much!!!!!Any help woud be great,thanks