However, I'm confused as to whether I should tell him that I'm in this lifestyle, of if he will think it is totally weird. Or, should I just let him think that he and I are having an affair?
Go with honest. If you are after dating him in a honest, ethical polyship, just TELL him you and your DH are Open, and you want to date him in honest, ethical polyship. Then see where the cards fall. Maybe he is Open to dating in honest, ethical polyship too!
But do not hook up with a married coworker just for casual sex and become his cheating partner. That's not honest polyamory. That is also not honest swinging.
From your intro
, you guys seem to want to swing -- recreational sex. Where the wife has sex play partners and the husband does not but enjoys his wife's adventures. That's cool -- everyone can have the right to have what they want to have for their relationship so long as nobody is getting hurt.
In this article
-- about open relationship models, it would be some sort of 1A arrangement between you. Is this model what you are after? If so, perhaps people can help you find resources.
But becoming someone's cheating affair partner -- that's not flattering to you and that IS hurting the spouse kept in the dark. You can find willing, honest sex play partners if this is what you wish to have without that kind of mess.