Thin wouldn't worry me, I don't know what you mean by he looks "weak". If he doesn't look healthy and strong, that's different. My first baby weighed 19 lbs. for his first birthday, he was perfectly healthy. My middle son weighed 19 pounds at about 5 months old, he was perfectly healthy, they both weighed about 150 pounds at 19 years old. (8.25 pounds is just over 18 pounds)
If it isn't weight, people will tell you he isn't walking soon enough, or talking soon enough. Ignore what other people say. Listen to your doctor, and trust your own instinct!!
at 10:06 AM on Nov. 15, 2010