It's sounds like a terminology issue. I think what he is telling you is that he wants 'enroll' with Medicare, but choose not to 'participate', in other words, not sign the participation agreement. As I understand it, by not signing the participation agreement, the rate paid will be 5% lower than the standard Medicare fee schedule. But non-participating providers have the option to not accept assignment. They cannot balance bill the patient, but it does allow the provider to collect up to the Medicare limiting charge (15% above the non-participating provider rate) from the patient, and then Medicare will issue their payment directly to the patient.
Providers can do this, but in my experience, it is a big administrative headache and also something that patients really do not like, so often not worth little bit of extra money that it can bring in. It equates to being able to charge 9.25% more than the standard participating fee schedule, but then you have to estimate and calculate the amount the patient owes and collect it in advance, so it can frequently result in credit balances that have to be refunded. In addition, patients' secondary insurance payers often do not understand how to coordinate their benefits correctly for this situation, which can also result in overpayment, or payment made to the provider instead of the patients, which then also have to be refunded.