What Breed Is My Dog?

I got my puppy named Maestro almost 6 weeks ago when he was 8 wks old and weighted 13 lbs. See the first picture. Now he is 11 weeks and 25 lbs in the second picture. His mother is a Boxer, Lab mix. They told us his father might have been a Mastiff. We are thinking that is true. Can anyone tell just by his picture?