I would hate to feel a "spark" with someone and THEN learn that he is polyamorous.
So I would recommend mentioning it up front on the profile. (I have lately become a fan of OK Cupid and have received compliments on my own honesty there).
He could also try seeking out women who are openly polyamorous and/or into group sex at whatever sites/places are best for that.
If he wears his wedding ring normally, he should leave it on. But I also think he shouldn't make dates with anyone unless he's already clear about being married.
Single, straight, female, solo, non-monogamous.