My lawyer told me it's better when AP is done from the requestee himself. It makes your application look a bit more personal and imperfect since you are not a lawyer. It erases the middle man(lawyer) and makes your application look more urgent and intimate to uscis.

The letter needs to be from the physician himself, such as his office or department or whatever, try to be as official as possible with this, maybe have him use his license number.

Have him state it to whom it may concern, describing the illness

You do not need to pay a lawyer for this . Do it yourself. Save money and a better chance at it being accepted