your doc probably will have no clue as to what Aikido is, and since you haven't started, you may not, either. Go watch SEVERAL classes. If you can, take a book that shows the falls (or be sure you can describe them), get a feel for how long and active the classes are, and the room temperature (some dojos can get pretty toasty in the summer), and the facilities availble if you need to use injections.
Pay attention to the class/instructors during the several classes you attend. Do they use a lot of rib atemi? with how much enthusiasm? Your will need to clearly describe the art to your doc if he is going to give a good opinion on your ability to train.
Good Luck, hope you start training soon.