One approach is to talk about polyamory in a hypothetical tense first, gauge his reaction, and then bring up the fact that you actually have an open marriage yourself. From there, it's just like regular dating... Flirt back, ask him out for coffee, the usual.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker