My daughter has this. It's not obvious to most people, except to nurses and doctors. I would encourage you to let yourself be a little sad, as that is perfectly normal and human. However, now is the best time to determine the way you will help your son and the rest of your family handle anything that may come up. Find a good balance. I think that's the most important thing. You will feel less sadness when you are informed, prepared and courageous.
By finding a balance, I mean: be informed about Pectus Excavatum, but don't drown yourself in searching it on the internet and don't become obsessed with reading about it. Believe me, it's easy to get carried away. Sometimes Pectus can be the only symptom, and sometimes it can be a component of another diagnosis. Each case is different, so don't jump to conclusions, but be alert. Don't anticipate complications, but don't put your head in the sand either. It's all balance.
I also mean, be willing to talk with your child and his doctor about the condition, but don't make it what your child is all about. Be honest, but don't make it his identity. Don't introduce your child and say "he has Pectus Excavatum" or bring it up in ordinary conversations. But if it increases as he grows, and if it affects him in any way, be willing to be an advocate for him and to teach him to advocate for himself.
Another way you'll need balance is if asked about it by other parents. Be up front about it, have an informed explanation, but make sure that you don't give the impression that it's a tragedy that is overshadowing everything. Don't just pass it off, either (don't say "it's nothing"). Be ready to say "yes, my son has a condition called Pectus Excavatum. It means that his sternum is deeper than most, etc." If they ask you if you're worried, or if they express inappropriate fear or other undesirable comments, be willing to be honest, but take this precious opportunity to educate them. Be ready to say "it's not who he is, or how his life will matter. That will come from a good character and qualities that have nothing to do with this physical difference. Please ask me if you have questions, but treat him like any kid." They may have legitimate questions about how it affects his health, and they way you explain it will determine how they treat him or any kid with any difference, whether visible or invisible.
Strength and wisdom and courage begin at home, and when we as parents have a child with medical issues, we have a very important and challenging goal: to establish how our children will face the world, how they'll deal with their physical limitations, and how they set and reach goals. My daughter sometimes is sad about all she has missed out on (she has multiple diagnoses and issues), but she doesn't dwell on it most of the time. Your son may not be the star football player or a long distance runner, but he will be something wonderful. Embrace it. My daughter has spent so much time in bed and alone in her room that she has learned to take a computer apart, add power and memory or do something else to it, and put it all back together. The other day I found her in her room taking apart something complicated. She had her computer, tablet and smart phone all showing different instructional videos and she was laboriously (and successfully and safely) repairing some complicated electronic thing. It surprises us every time. Be prepared to be proud of your son! Expect it and show him that you will support him through the tough times and the victorious times.
There's a girl who was recently on the X factor tv show, and she's 13. She has a very serious and obvious joint deformity in both hands and arms and partial blindness (nothing to do with Pectus - another condition altogether) and her smile and talents are amazing. I don't remember her whole name but she has an unusual first name - Rion. So if you google Rion X factor you will see her story. Her mother went to karaoke and other singing competitions with her to hold the microphone because the girl is unable to. And her family has shown her how to feel valued, and courageous, I think her story (she's been interviewed several times) is a wonderful example of parent and child pulling together, with a good balance between the realities of the medical problems and a life being lived triumphantly.
Feel free to send me a message if you have questions about Pectus. I'm proud of you for being willing to be sad but also willing to seek encouragement.