To minimize the taxes in retirement, you have to do very calculative thinking.
If your father is still working, tell him to contribute more to traditional IRAs and 401(k)s (both are retirement plans that offer tax benefits); this will reduce his current taxable income.
To keep up with inflation, select a long-term growth investment plans and tells him to add dividend-paying stocks to boost the portfolio.
Transfer their pre-tax retirement savings to a Roth IRA, as Roth contributions aren’t tax-deductible, regardless of your income.
Consult with an expert. He will create a personalized plan for your father according to their savings, income, and tax plans.