The focus should be on if he's a) healthy, and b) gaining weight in a healthy fashion----not how much he weighs or how he compares to other babies. Some kids are just small/thin, and that is all there is to it.
Knowing how much he weighed at birth, and how much he has gained since then at his various wellness check-ups is key. That's why your pediatrician weighs him and measures his height & head size.
The easiest thing to do, rather than worry, is call and talk to the nurse. They can pull his charts and tell you what his weights were at various check-ups, and how he is measuring.... What they typically want to see is that he's either staying at his percentile or is gradually gaining. They don't want to see him drop in his percentile, nor make a huge jump up either.
For example, my son was small (6#10oz) at birth, and was at the 10%. Over the course of the next 6 months, he gradually moved up to the 25% percentile, and that is where he has pretty much stayed, for weight. He started at 50% for height, but shot up to 75% by 6 months, and has stayed there. He's 27m now, and still wearing 18m pants because he's skinny.
So, rather than worry, can your doctor's office and speak to the nurse or doctor. They may want you to bring him in, just to be on the safe side (it's always better to catch any issues early, especially in babies).