K.M.
My 4 year old has a sunken chest, but it was caused by 2 open heart surgeries--that's just how his chest/sternum healed. I asked his cardiologist about it, and he said it was normal for kids in his situation--sometimes they heal and cave in and other times they stick out.
I would imagine that your son still has a lot of growing to do, so it's quite possible it will change. Perhaps you can call your doctor ahead of time--or after the appointment--to discuss it. I agree that you should not talk about it in front of your son--mostly because he's never complained about it:)