IMO if the pediatrician thinks all the child needs to do is to eat until satisfied and is getting the proper nutrition, I wouldn't worry. He'll come into his own in time, believe me. We raised 4 sons and a daughter and, when the time comes, they'll eat you out of house and home.
Seriously, though, as long as he is getting the right balance of foods, I wouldn't worry about him.
And, a someone else queried, who
judged him to be underweight? Friends? Family? Many years ago, it was thought that a "fat" baby was a healthy baby. Wrong assumption. It would be helpful if you could give us a little more background information. Still, his doctor is the best source of advice.