Hi Krysten and glad you have found your way here! There are good people and good advice around.
Just because he says now that he can never ever share you in any romantic way at all, doesn't mean it shall always be like this. Keep the lines of communication open and focus on building trust and mutuality in your relationship with him. Being in love is an urgent feeling but from what I understand, Alan isn't going anywhere.
Think back on when you started swinging. Was it a situation that you just arrived at in a "I would like to bang our neighbours on the side", "Wow, that sounds awesome, let's do it!" kinda way? There was probably some negotiation involved, certain boundaries that were established that you more likely than not needed to tweak a bit once you became more experienced etc? Are you assuming that because things have gone so "naturally" up until this point that they should proceed in that manner?
I don't suggest you start leaning on him heavily and manipulating him into polyamory. People can change their minds - go from swinging to poly to mono and back again. You've asked a couple of times, and he has said no. Where's the rush?
Me: bi female in my twenties