Oops, I missed the first part about him being eligible for Medicare:

Notice 2004-50:

Q-2. May an otherwise eligible individual who is eligible for Medicare, but not enrolled in Medicare Part A or Part B, contribute to an HSA?

A-2. Yes. Section 223(b)(7) states that an individual ceases to be an eligible individual starting with the month he or she is entitled to benefits under Medicare. Under this provision, mere eligibility for Medicare does not make an individual ineligible to contribute to an HSA. Rather, the term “entitled to benefits under” Medicare means both eligibility and enrollment in Medicare. Thus, an otherwise eligible individual under section 223(c)(1) who is not actually enrolled in Medicare Part A or Part B may contribute to an HSA until the month that individual is enrolled in Medicare.


https://www.irs.gov/irb/2004-33_IRB#NOT-2004-50


But as was mentioned, beware of the 6 month retroactive coverage.