Hi P.,
I am a mother of 6, soon to be 7, and my babies have all been drastically different as far as growth. My first was 20 pounds by 6 months, but my sixth is only 24 pounds at 22months. However, even though my sixth is so tiny, he has been my healthiest child. He teethed very late, walked later than the others, wouldn't eat anything for almost 15 months (just nursed), but there was nothing wrong with him. He's happy and healthy. I wouldn't immediately discount what your pediatrician says, but I wouldn't just accept it either. Someone's gotta be in the 2nd percentile, right? Is your baby really underweight, or just not a big baby? I would look more at whether he is growing, whether he is generally healthy, and whether his general development is within "normal range." You'd be amazed at how much a baby can eat without choking, even without many teeth. As long as you are right there, you can keep him from choking, as long as he's not eating something like peanut butter that sticks in the throat. I'd watch his weight gain, but I wouldn't worry too much unless he's losing weight, or is way off schedule with developmental milestones. You know him a lot better than your doctor, so don't be afraid to question their "expertise." Good luck.