I too am not familiar with Mastiffs as far as weight is concerned.
Could you speak to your breeder and find out what you should be expecting as far as his development is concerned?
Our Saint Bernard Norman is 11 months old this week, and tipping the scales at 150 at the moment and you can still feel ribs quite easily.
We are feeding a raw diet, so we too drop in at the vet regularly for a weigh in and to make sure that he's not gaining too fast. So far so good.
We spoke with his breeder last week, and she mentioned that she had had one of Norman's sibblings in at the vet and he was weighing in at 120 lbs, so compared to his brother, Norm is bigger.