Just some quesions for further clarification:
Do you know what his idea of poly is? What does he envision for himself?
Why did the topic came up now? Is there a new interest of his already? Or is it more like a general feel he got that he is missing something?
Did you two already talk about the shortcomings of your relationship? Before the poly topic entered the picture?
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.