What type of specialist should we see for a 15 yr.old male with a subluxated first rib fracture?

Probably none: Rib fracture should heal on its own and there's not much you can do for them. If it persists over a long period of time, potentially a thoracic surgeon could do something but this would be very rare.

Thoracic cardiovascu: Most heal as has been answered, but concern regarding any other ribs, and any nerve or vascular impairment arises. The thoracic outlet / first rib area is adjacent to major important structures. Thoracic cardiovascular is the discipline to see usually.