One more point to mention is that repaying a mortgage now, then selling the home, might not release much if any cash as cash. His OA, plus accrued interest, would be repaid first. And since he evidently didn’t meet the minimum sum for his cohort then I wonder whether that OA could even be withdrawn in a lump sum....
....If that’s how it’d (not) work, that seems like a much worse roundtrip than just depositing funds directly into his RA — with tax relief, I should also mention. It seems fairly reasonable to assume that a 73 year old parent with basically no income and no assets to his name except MediSave and HDB leasehold equity ought to sell off his HDB leasehold equity to pay for food, electricity, clothing, soap, etc., maybe now, especially if the home is too big in the circumstances. So I’d be very seriously considering the Silver Support Scheme with him, or a simulation of the Silver Support Scheme if he’s not actually eligible for it.