The most likely source is the Medicaid program in the state where your brother-in-law lives. Medicaid can cover not only medical care but also some home care, including personal assistance with the activities of daily living. If your brother-in-law is eligible for Medicaid, it will also greatly reduce the cost of his medications.
Medicaid pays the full cost of residence in a participating nursing home but not usually for an assisted living facility. However, in many states, the Medicaid program can pay for part of the cost of care in an assisted living facility. This is most often true if the person receiving the care would otherwise be eligible to receive Medicaid coverage for nursing home care -- in other words, if the person needs what Medicaid defines as nursing facility-level care, and if he is not able to remain in assisted living would have to be moved to a nursing facility.
Whether there is any coverage at all for an assisted living facility, what the coverage requirements are, and whether your brother-in-law is eligible, all depend on the specific rules of the state where he lives (Medicaid rules differ from state to state). To find out the specific rules regarding eligibility and coverage in an assisted living facility in the state where your brother-in-law lives, go online to any search engine and enter "Medicaid" and the name of that state. Or, you can call the Eldercare Locator toll-free at 800-677-1116 and ask for the number of the state's Medicaid office.