Can you suggest how to get my 21 year old son seen by a doctor who might recommend the nuss procedure for pectus excavatum?

See plastic surgeon. Don't look for a specific procedure, but see an expert who can recommend a number of options that can best treat the exact type and severity of pectus excavatum your son has. He is an adult and he has a voice in this as well. Present options, not pressure, as this is almost always cosmetic rather than functional. See plastic surgeon at a major university or teaching hospital who has done many!
Ped surgery referral. Most Nuss repairs done in patients your son's age are done by pediatric surgeons. I would recommend arranging a consult for him. The closest center of excellence for pectus repair near Knoxville is CHKD in Norfolk, VA ( Studies that can be done prior to consultation include chest CT scan, echocardiogram, and pulmonary function tests. These can be ordered by his primary care MD.
Thoracic surgeons. And some plastic surgeons are expert in this diagnosis and the treatment concerns. Ask the local medical society for a list- or google. He has probably passed the best timing. Get a consult.