What Causes Skin Darkening On Lower Back Rib?

my 13 yr old son has a blackening on his lower back rib. it started like from one bone and it s getting up to almost 4 bones. he also cannot stand straight. we don t know if this is a result when he fell down from a 4-layer cfair when he was still 8 or 9 ys old. please help us what is this