This may be a result of lifting some heavy object that put stress
on the muscles.
He is having pain due to muscular spasm or due to some inflammatory process of the costal cartiliges called as costochondritis
He should have an xray chest to rule out any cause in the lungs. If the lungs are fine take pain killers such as Ibuprofen
400 mg thrice a day for 07 days if he is having any stomach issue than use Cap. Omeprazole
40mg once a day in the morning before breakfast to prevent any heartburn
I hope I am sucessful in solving your query If you have more you can ask me.
Dr. Azhar Sattar