Well they can make a deal along the lines of 'give back those shares or we'll issue millions of new shares and give them to everyone we like except you'.

He owned <10% at the time so it’s a little bit of a mischaracterisation to say “most” as if it means he lost control because of it. He did not lose a meaningful amount in the context of a conversation about control, he lost a few % of the company.

I think he meant "my shares" as in the shares that he personally had at the time.

Confusing how they could take away shares he already owned and had fully vested.

Was that the context?

Most likely unvested shares