My 3 month old baby boy...?

My son is 3 months old and weighs 12 pounds. He was born 2 weeks early and weighed 7-6 at birth. He is the smallest of all my babies and I am forever finding that I am comparing his weight to other babies. It is crazy, and I can't seem to help it. he seems so tiny to me..even though my family tells me how big he is now and the doctor says that he is very much in proportion and normal for him. He has reflux and so sometimes dosn't feel like eating much and it drives me crazy. I keep thinking that he is going to stop gaining and wilt away. Has anyone else had smaller babies and how did you cope with strangers telling you how big their babies are and how small yours is? My son is formula fed due to my milk not coming in and I feel guilty for that. I fed his brothers so I feel a lot of guilt for not being able to feed him.