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
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