Deduction U/s 80D

Certainly yes.
does he receive pension .
has he retired from govt. service.

Yes, he can claim deduction under section 80D

Yes the assessee can claim Deduction U/s 80D of the Income Tax Act 1961 for the Parents Mediclaim Insurance premium upto Rs. 50,000 ( if they are Senior Citizen )...

Health insurance premium paid for Self, Spouse or dependent children is tax deductible upto Rs 25,000. If any one of the persons specified is a senior citizen and Mediclaim Insurance premium is paid for such senior citizen then the deduction amount will be Rs. 50,000 for FY 2019-20 (AY 2020-21).

Health Insurance premium paid for parents is tax deductible upto Rs 25,000. If your parents are senior citizens (60 years and above) then the maximum allowable deduction is Rs 50,000. For senior citizens above the age of 60 years, who are not eligible to take health insurance, deduction is allowed for Rs 50,000 towards medical expenditure