The states of NY and CT are year round guaranteed issue and he cannot be denied Medigap coverage. Outside of those states, he will be subject to medical underwriting. Depending on the insurer, the Medigap may deny the application, accept him at a higher rate but not cover the surgery, or not impose a pre-existing condition limitation. It can depend on if the upcoming surgery is scheduled to be outpatient or inpatient and if there has been a recent inpatient hospital admission.
It is currently open enrollment season when those on Medicare can switch to Medicare Advantage plans without underwriting. They will have provider networks, copays, and coinsurance. I suggest contacting an experienced local insurance agent to discuss your options. Should you go the Advantage route, a local agent will be more familiar with how comprehensive their provider network is in your area. Good luck.