In that case, maybe his OPT employer offers to register in the annual H-1B lottery next March and is drawn, so the employer can petition for an H-1B visa him. Or maybe the employer decides to sponsor him for an employment-based green card.

If he does date and marry a US citizen soon after divorcing you, that could look a little suspicious as to the genuineness of the new marriage coming so quickly after a divorce and while the clock is ticking on staying in the US. But that's just as to credibility factors a USCIS officer might consider, not a hard-and-fast rule against another marriage and an immigration petition / Adjustment of Status application.

Disclaimer - all of this is general information only, not legal advice. He (and you) can consult a US immigration lawyer for legal advice about the situation.