How do I know what information a doctor should disclose to an employer about his patient's physical limitations and need for accommodation?

My son has a heart condition. He works for a grocery store. Part of the job is to push baskets in from the parking lot. My son Chris is sometimes told to perform this task for over three ours and sometimes for most of the day. There are other types of functions in the job description, the major one being to sack groceries and to help customers to their cars with the groceries. Chris has worked for this store for over 5 years and has not had a problem with any aspect of the job until this year. Management has in the past been reasonable in their expectations as to the amount of basket pushing any one individual must do in a given day. We gave the human resources manager a letter from Chris' Cardiologist when Chris started working there 5 years ago and have given several to various store managers over the years. The current manager does not think the cardiologists letter was specific enough to warrant allowing Chris to take a break from pushing baskets when he is tired and winded. Help.