My son will be 1 month old and has only gained 12 oz. of weight is this normal?

My son will turn 1 month old in 3 days and hasn't even gained a full pound yet. Is this normal? Is he under weight? He was born at 7lbs 5 oz. and weighs 8lbs 1oz He is 100 % fed and wets and dirties diapers regularly. He appears to be healthy no colic, no spit up, no vomit, diarrhea etc. Sleeps 22 hours a day. Shouldn't he have gained more weight though? He is only getting longer but not meatier. He also has very little body fat.