It sounds like you are asking a simple question. One way to start the process is to ask the social worker at your son's school about the resources and policies in your area.
Residential services vary a great deal from one state to another.
Sometimes there are long waiting lists that can take up to several years. You may need an advocate or an attorney to help you unravel the red tape. You may receive contradictory information. The pendulum has swung back and forth on this issue. Before the advent of federally mandated special education in the least restrictive environment, parents were routinely counseled to place their children with severe disabilities in institutions at an early age.