Thanks for your query.
Since three doctors have diagnosed a herniated disc
I shall agree with the doctors.As for the chiropractor saying pinched nerve
let me tell you that a pinched nerve more likely happens when a person has herniated disc on any of the side.So in either terms your sons problem and even the treatment remains the same.
I would suggest you to consult a Orthopaedician for opinion and treatment as I am unable to see the MRI
report so difficult to give an opinion.