When should I be concerned that my 14 month old isn't eating "enough"?

I was told by his doctor and several other people that it is normal for a toddler to go through a stage when they just don't eat very much. Well, my son hasn't been eating well for a while and has been dropping in the percentiles. He gained barely over a pound in 3 months at his last doctor's visit. She said as long as he's still gaining that it is good. He is extremely active and has non-stop energy. I can barely keep up with him. He leaves most of what I give him. I don't force him to eat. I let him eat how much he wants. It seems like he has gotten thinner recently, his pants are looser at the waist. He wasn't very chunky to start with. At his 12 month appointment he was 30 inches long and about 23 pounds, which is right in the middle for percentiles. He was at one time in the 75th to 80th percentile though. I have a friend who has a baby that is 6 months younger than my son and they are about the same size. I'm just wondering when I should be concerned. He was such a voracious eater when he was a young baby.