My father is 82 years old and currently in a rehab facility for sub acute care. How can we get them to let us move him and still be covered by Medicare?

We have made arrangements for him to be transferred to an assisted living facility for continued care so he can be with our Mom. The assisted living facility can provide PT/OT. The rehab facility is now saying Medicare will not cover his care once he is transferred. I feel like the rehab facility is trying to milk his Medicare reimbursement for their benefit instead of letting us transfer his care to our chosen place- how can we get them to let us move him and still be covered by Medicare?