4 Month Baby “overweighted”

My Baby Was Born On October 16 Born at 37 Weeks Weighing at 6pounds 10oz. Here At 4 Month Hes Weighing 18 pounds exact. My Doctor has Said His “overweight “. Got A Little Worried About The Idea Of Having My Baby Having trouble learning to move around . My Doctor Said He Can Have trouble Lifting His Head Or Turning Around Because Of His Weight. He’s Head & Neck Is Still not strong enough to hold His self but He Tries And Gets Tired. He’s a healthy baby boy eats 4 oz formula but I still breastfeed him sometime But he never is satisfied with my milk & during the night sleep all night. Im I Doing Something wrong?