Hello welcome to the health care magic
Your son has just started exercise...
Hence according to history it points towards muscular strain pain induced by exercise
Before labelling it as
muscular pain I need to know... Following details...
1.
chest pain present in resting position??
2.fever??
3.cough present??
4.
Smoking history??
For muscular pain
ibuprofen or
Naproxen prescribed for five days
Warm compress beneficial
Better to take rest for few days
Start exercise under trainer and start with light exercise
Avoid strenuous work for seven days
If improvement not seen then I will suggest chest x ray, EKG, CBC investigations and general examination
Hope I have solved your query
Consult nearby doctor for Auscultation and examination with keeping all this in mind
Provide above answer for giving more comment