Training your chest too frequently and neglecting back will make your pectus excavatum look worse.
Pectus Excavatum sufferers already have very tight chest muscles & when you train chest with weights you make these muscles even tighter. These tight chest muscles pull your posture forward and contribute to the kyphotic hunched position which makes the indent appear deeper.
Training back will do the opposite, it will straighten your posture & will be very beneficial for the correction of Pectus.
However, I do still recommend training chest as developing the chest is also key for improving pectus through exercise. Just be sure to train the back equal to or more than your chest and also to stretch your chest thoroughly. You can check out this article on stretching for pectus
Some great exercises I recommend doing to develop your back and thus improve your pectus excavatum by improving your posture include.
These exercises in conjunction with an awareness of your posture and a daily effort try and form a habit of standing and sitting up straight will do wonders for your case of pectus.
Watch this video below to learn more about why training chest too much can make your posture worse:
If you want to improve your case of Pectus as much as possible then signing up to my programming would help you a lot. You can learn more about it at www.fixpectus.com/the-transformation-project.