You may be responsible to reimburse the Government if he uses certain means-tested benefits. Go to www.uscis.gov to understand your obligations.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.
Medicare is only available through SS retirement or SS disability. Also available to legal permanent residents who are 65 or older and have been LPRs for at least 5 years. Although the facts you provided are limited, your dad does not appear to be eligible yet.
[This answer is for general purposes only; it does not constitute advice and does not establish an attorney-client relationship.]