Even a doctor who does not "accept" Medicare can see Medicare patients and help you get the fees at least partly paid by your plan. You might have to pay his fees from your own pocket, and then get paid back by Medicare. His office should submit the Medicare claim for you and they can't charge you for doing this. You can submit the claim to Medicare yourself. Medicare has a claim form on its website: Form CMS-1490S.
Since he is not "participating" with Medicare, he is allowed to charge you up to 15% more than what Medicare pays to non-participating providers. This 15% limit does not apply to all services, however. You can call the plan to find out the details.