Hi, My male English Mastiff is about 5 1/2 months old. He weighs about 85 +/- lbs. He weighed 78 1/2 lbs when he 5 months old. I am trying to figure out how to post a picture of him but and having no luck. If I figure it out, I will go back in and post one.
From talking to my vet and other people who have Mastiffs, it is important you don't try to grow them too quickly or they risk developing joint problems. I am feeding my dog Eukanuba Puppy Formula for large Breeds. I fill his bowl twice a day. In addition, I slip him a milk bone here. My vet did tell me it is not unusual for these dogs to be gaining 1/2 lb a day during this stage of growth. If he is skinny and leggy looking, a bit clumbsy, and sometimes stiff when he wakes up.... I think all of this is normal growth. Mastiffs will grow for two years. They get tall and lanky first, then they start to fill out....
Hope this helps.
(this link takes to you to some pics of Texas, my mastiff.... by the way, I am also a NY Giants fan) http://boards.giants.com/forums/2/881237/ShowThread.aspx