I looked at his pic and I don't see any bull mastiff. His nose is a little long though. That doesn't mean he is a mix, just that he doesn't come from high quality, show lines...and since I assume you aren't planning to show him, that doesn't matter. What does concern me is that he was removed from his mother and littermates at 6 weeks old. Unless he was the sole survivor of his litter and his mother also died, there is no reputable breeder alive who would condone placing a pup at 6 weeks old. It is actually illegal in many states to remove a pup before 8 weeks and many reputable breeders prefer to wait 10-12 weeks. There can be serious behavior ramification to removing pups so young so you will want to be on the look out for issues with bite inhibition as he gets bigger.
I feel like you are getting a little defensive and please know you are not being attacked. Many of us have bought pups we thought came from great parents and breeders only to have a myriad of different problems. I did all my research and got our last pup from a show breeder with all of the right credentials. I really wanted a pup from proven parents but everything looked so good that I took a pup whose dad was a first time dad. And, he had MANY health problems in his first year of life including his first malignat tumor at less than 1y/o. So, the info you are getting from people here is not to criticize but to educate not only you for the future, but for others who might be reading this thread.