I wouldn't worry too much if that is the general stature of your family. Is he mobile (crawling, cruising furniture) yet? If he is that would also be a contributing factor. I know DD basically stopped putting on weight at all once she started crawling, because all their energy is going into moving rather than growing. But that's ok, it will even out and they will catch up. Unless there's an underlying reason to be concerned, or e is actively losing a lot of weight I wouldn't worry just yet and just keep monitoring.