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.
Gralson: my husband (works out of town).
Auto: my girlfriend (lives with her husband Zoffee).
The most dangerous phrase in the English language is "we've always done it this way."