Although the medical is not a requirement to begin ground training, it is always a good idea to get your student to an AME as soon as possible. You don't want to have him spend a lot of money on ground instruction if he can't pass the medical.

It's not easy to tell someone that you can't refund them the money for the ground training that they've completed after they find out that they can't pass the medical.

It's always good practice to have them get the Student Pilot/Medical Certificate before training.