Pericarditis is mistreated (and misdiagnosed) by the overwhelming majority of physicians, including cardiologists. Often, the diagnosis is made because the patient has chest pain and an ECG which resembles (but in fact is not) the typical pattern of pericarditis. In fact, many young healthy athletes have a pattern on ECG called benign early repolarization which, to the untrained eye, can mimic pericarditis. This prompts the use of steroids (a major mistake early on) and possibly NSAIDs/colchine (better treatments).
A more thorough diagnosis requires TYPICAL chest pain (not just any chest pain), a "rub" heard on cardiac auscultation, elevated inflammatory markers on blood tests, and imaging (ultrasound, cardiac MRI).
The cause is almost always idiopathic and likely due to a viral (generally not bacterial) infection such as respiratory viruses or stomach viruses.
Exercise is thought to be deleterious to the healing process though the literature on this is soft (mostly expert opinion from pericardial clinics at top heart hospitals). This is particularly tough for centro as a professional athlete.
He should almost certainly be seen by an expert in pericardial disease as well as a sports cardiologist. The people who qualify for those descriptions are likely numbered on a single hand.