aptamil pepti

My little boy is 10 weeks old as has been pescribed aptamil pepti they give me 6 small tins and he said that should last 4 weeks well it's not going to he is drinking 5oz every 3 hours and we have used 3 tins and are on to our fourth and he has only been on it a week and a half do you think they will be ok with me ordering more and the end of this week or will they question me why he has drank so much he used to go through a tin and a half a week of normal week