Dont worry like everyone here already has said, it can take some time before they come around and act "themselves" dont baby him too much, because that can lead him to think he is right in his behavior, and that there really is something to be afraid of. So give him a little time. Mind you, I would also like to here say that a mastiff who is showing a bit fear and tendancy to be worried, well as soon as he comes around at home and is okay with you, start to take him out, work lots and lots on him being social, take him to parks, to petsmart and everywhere else where he is welcome. You want to introduce him to as many situations as possible, not too fast or so that it scares him, but you dont want a 220 lbs mastiff boy who is afraid of everything
Please post pics of him. I have a mastiff boy myself, Galahad who is about 11 months old now, and a lover boy
Good Luck and let us know how things go, or if you have any other questions
Marit