HollbillyNurse,
Wow, that's a very large osteochondroma. Most are not anywhere near that size. And the chest is not a common place for these to occur, as for an osteochondroma to form, there must be a physeal growth plate (either epiphyseal or apophyseal). So, they are most common on the long bones, then the pelvis, but can occur in any bone preformed in cartilage.
It is interesting that they used plastic replacement of the ribs. Usually in children, when the ribs are removed, subperiosteally, the periosteum is closed and the ribs will reform within that sleeve. So, in surgeries where the ribs are removed, such a thoracotomies or scoliosis surgery, the ribs are allowed to grow back naturally.
Osteochondromas only come back if a small portion of the stalk or cartilage cap is left behind. So, the tumor probably did not return, but he could be having problems from the Gortx or ribs. He could have developed adhesions from the surgery, and the foreign bodies.
He probably needs to see a cardiothorcic surgeon for an evaluation. Good luck.