Estimated weights of american pit bull puppys at 13 wks.?

i adopted an apbt/mastiff puppy at 7 weeks..he was 7lbs...skin and bones ...he's 13 wks now and weighs 26 ibs...not stout/stocky....just tall.....any way i'm trying to figure out if he really has any mastiff of any kind. i'v owned 2 english in the past definantly not that.....they said bull mastiff....showing know traights of that breed as far as i can tell..