Last year, my husband and I fell in love with a woman we met online. Initially, I was the one with feelings. He started off telling me he couldn't even imagine having a crush on another woman, let alone loving one, because I gave him everything he needed. He wasn't wrong about the latter part, but he did ultimately fall in love with her.
It was a long distance relationship, and the circumstances under which we met led to it being emotionally intense. We moved in together after three months. I do not regret that decision.
I struggled a bit, though, with accepting that my husband loved another woman, particularly given his original position on the matter. I loved her, too... but he readily admitted to me that he could never handle me with another man. Not that I'd want another man, anyway. The idea of any man who isn't my husband touching me makes me nauseated... but I digress.
I have a couple of points I want to make. One is that, in our case, I was romantically involved with the third person, and it was STILL very, very hard at times. If you aren't truly poly - and it certainly doesn't sound like you are - a poly relationship can hurt. A LOT. Now, there was a period of time in which what I got out of the relationship shared between the three of us made it worth fighting through my insecurities/jealousies and the hurt.
They tried to avoid hurting me. They never did anything intentional. But in these sorts of relationships, with the complexities, hurt is much harder to avoid.
You will hear the term "compersion," if you haven't already. It means that one partner is made happy by knowing his partner's relationship with someone other than him brings the partner happiness. I've experienced compersion, but it's not a simple concept, and it can be part of a myriad of emotions. I've been truly happy for my husband and girlfriend about things they've shared... but I've also had my own mixed feelings at times about those things.
This is a complicated lifestyle. I would definitely not move this guy in, primarily because you have a young child. Let him move near you, if you decide you want to let your wife pursue a relationship with him. Give everyone time to know each other... to see how he is with your child. To see if he and your wife even have the same feelings for each other when they're together in the real world.
I wish you the best...