My wife began her relationship with Charlie in earnest about five months ago. In that time I have had a few opportunities to date but I've chosen not to. The reason is pretty simple for me. I have two criteria for engaging with someone outside my marriage. 1. I will not do it as a reaction to my wife's relationship. 2. I will not do it at the expense of the person I'm interested in. That is to say that I while processing the crush of emotions I'm feeling, I find it better not to involve more potential instability. Also, I can't really see myself being very attractive to anyone while mired in jealousy. Not very sexy. As luck would have it though, I feel like the haze of uncertainty is lifting. RC and I have worked very diligently on ourselves, our relationship(s) and making each other feel as loved and supported as possible.
Allow him time and support. Maybe introduce him to all the lovely folks on this forum. Be yourself and let him do the same. I would also avoid throwing around labels like mono and poly just yet. Let things happen naturally.
Al the best to you both.