what I might do
Since I'm new to being poly please don't take what I write as anything more than what I might do if I found myself in your situation.
I personally would hold back a little until I knew that his wife will not reject him having an emotional relationship with another person. Because it could be that she says no and he decides to withdraw from the relationship he has with you.
That's just me and it takes me a long time (6 months or so) to trust people or even trust my own feelings and think that it may be love.
I wish you luck on your journey and welcome to the forum.