Anyone have experience with "pigeon chest"?

If his chest is sunken in, that is pectus excavatum, NOT pigeon chest. Sunken chest can be a sign of Marfan's - is your son also very tall? Even if it isn't related to Marfan's for him, it can lead to heart and lung problems. My friend whose son had this (and had to have a couple surgeries to correct it) also had a heart attack at 21 due to the damage his sternum and ribs caused his heart.

I would strongly request that the pediatrician refer you to a specialist at this appointment. It really is nothing to dink around with.

Yes, sunken chest is called pectus excavatum. DH, DS, and DD all have it. None of them have Marfan's, though they have had cardiac echos to confirm that there is no extra pressure on their hearts. My children are very young, so it hasn't affected them yet. But it can become more severe as the hit puberty, when the rib cage expands, the dip in the sternum can get deeper. There are cosmetic treatments, such a Nuss bar placement but they are fairly painful and involved, the Nuss bar in particular is known as one of the most challenging recovery processes for children, so I'm hoping they won't be interested in that option as they hit puberty when it becomes a cosmetic option.

DH has lived with his for his whole life and it rarely comes to mind now.

My brother had a sunken chest as a child and well into adulthood. It's not as pronounced now, but as a child, he was very thin and his poor little ribs showed because of the sunken look. Nothing was ever done in terms of diagnosis or corrective treatment, and he's had a perfectly normal life in terms of health. He was a burly truck driver until he retired, never had lung or heart issues, and is now a relatively healthy 65 year old man. The only real health problem he's had is back issues due to years of bouncing around in a truck, loading and unloading merchandise, etc. So, not necessarily pectus excavatum.