What should my 15 mo. english mastiff be eating?

i have a 15mo. mastiff and im not sure if hes eating enough. hes not really skinny but his belly looks small and you can feel his ribs. right now i give him 2 scoops of adult dry food ( vet said to give him adult after 6 months) and and a full can of wet adult food. last time we took him to the vet which was a while ago he weighed 125 lbs. and when he is full grown he should be between 200 - 210 lbs.