If he kept the shares until he died, his heirs would have gotten stepped-up basis so there would have been no capital gains anyway.
Of course, a savvy parent would put pre-IPO stock into a trust, using up some portion of the $5-10 million exemption amount (it's up in the air because some of the Trump tax cuts are not permanent). Millions of dollars in pre-IPO valuation stock turns into tens or hundreds of millions by the time you die. And there are more nuanced (and/or aggressive) strategies that can yield greater results — or at least that's what I remember from my trust and estate law class.