Hi and thanks for the query.
The only thing that is the most concerning in this scenario is a cardiac event which at his age is extremely unlikely. If this were a cardiac event or something serious, that is a cardiopulmonary pathology, he would have had shortness of breath, bluish discoloration of skin, cold sweats, confusion, excruciating chest pain sharp and dull going to the back, left arm or the jaw, there would have been blackouts or fainting, he would be very anxious as opposed to being calm and he would have had a prior history of having such symptoms as well.
This is most likely a muscle strain, these are very painful as well, as the chest is a dynamic area and has to move all the time with breaths.
His symptoms do not warrant an ER visit however, since I have not physically examined him, if you seek peace of mind an ER visit can be made nonetheless.
To ease his symptoms, an OTC pain killer and a massage with an NSAID liquigel on the affected area with bed rest should be beneficial.
However, if you see that his anxiety is worsening or his symptoms are aggravating then take him to the ER to be on the safe side.
Let me know if you have any other queries related to this.