Do you know how his friend feels about all of this? We have many friends that would love getting to kiss a man's wife. They'd even more enjoy having sex with her. But, most have no desire for an emotional relationship. Keep in mind we are swingers, but just because this man kissed you doesn't mean he is interested in an involved relationship. A poly relationship for the single mono to join could be daunting to him. I am not trying to rain on your parade at all. I am glad to see you are working together on all of this and the conversations are important, just one more thing to think about with it... Do both of you (and the other man) want it poly vs swinging?
Best of luck, you're in a good place!