It sounds like yoiu have accepted the idea that you can love two people at the same time, so you got the basic poly philosophy. (The other component is making sure everyone is informed of what is happening.)
I think one question you should ask yourself is if your boyfriend did open up emotionally, do you think you would still love this other guy? In other words, is he fulfilling a strong need and there is not much else attracting you or is he overall someone you are interested in.
I think you should talk to your boyfriend about the concept of polyamory and see what he says as a first step. Before you do anything with the other guy, you need to be on the same page with your boyfriend. It is hard to recover from a hidden affair (though it is possible).