If the business is >1 year old, it's treated as capital gains. Since the company is >5 years old, he can likely take advantage of the Qualified Small Business Exemption up to $10m and pay no federal taxes.
To be specific, since it might help someone and it’s important to get the technicals right, dig into the IRS portion of the federal code, section 1202, or QSBS. Note, doesn’t apply to LLCs ... so you eat double taxation if cash flow positive but it’s less burdensome now with the Trump tax rule.
I believe, but can’t find, that you need to trade cash for the shares to qualify, so if your basis isn’t zero, it might require writing a check into the entity to cleanly qualify.... moral of the story here is pay someone to help you on this :)