My daughter is doing the opposite – pectus caranitum, so I have read up on this a bit. That caving in is called pectus excavatum. It is EDS/collagen related. But, it is not a big deal unless it is actually causing difficulty breathing or compressing the heart. The psychological issues with a different appearance can also be significant, as kids get older.
There is surgery that can be done. For the pectus caranitum, there is bracing to compress the bulge, but pectus excavatum would require surgery if it needed addressed. There are some exercises to do when it is mild, but the info I read indicated that they are not very successful.
Like I said, from what I understand, it is not a big deal but you probably want to bring it to the attention of your EDS doc. They can keep an eye on it, particularly if it is progressing rapidly. FWIW, my daughter’s is progressing rapidly too and it is quite painful. But, it is not something to get really worried about. Although, that is easier said than done and I should take my own advice! 😉