HSA

To qualify for an HSA you must be enrolled in a health insurance plan that imposes high deductibles.  You can use previously contributed funds.  Unfortunately Medicare recipients do not qualify. 

https://turbotax.intuit.com/tax-tips/health-care/what-is-the-irs-form-8889/L8hRNHx4o

If it is your son and you are asking on his behalf he must be enrolled in a  high deductible health insurance plan.

Following is a good source of information on the specifics: https://www.irs.gov/publications/p969