my ds was born at 6pounds 15 by12 weeks he had doubled his birth weight to 14.2 pounds. so it sounds like your ds has done the same over a slightly loner period of time he sounds fine to me. of course if your baby was born a good weight he will be heavier now. at 4 months ds was 16 pounds if that helps reassure you my ds is now 9 months and around 22 pounds and perfectly fine on around the 75th centile. your ds weight gain will eventually slow right down. remmeber your baby is growing at the quickest rate of his life right now.