What Causes Pain In Chest, Neck Veins And Arms During Running Long Distance?

My 16 year old son is experiencing some progressive symptoms during his run - chest pain, left arm pain and the veins in his neck get very painful. His heart pounds too. He has been a regular runner for almost 6 years so these are not usual. He feels like a small part of it now is anxiety but we are having a hard time getting people to take him seriously so.... any opinions?