Noticed many reads but no replys to your thread; know that there's usually a lot going on all over the forum so posts sometimes get passed by. Hopefully one of the long timers will chime in soon, they so often have such helpful hints and experiences to share.
Imagine you left out many details in keeping the post concise (well done); It's so amazing that you seem so ok with the major transition of your relationship. Wonder how long that took? I can relate to many of the details of your experience: wanted you to know you're not the only one. Now that you're moving forward.... a little possible help?
With dating, my husband was directed to OKCupid; a dating site that lets people explain more about where they're at and what they're looking for. My husband's profile says he's in a relationship with his wife and her boyfriend. It says "available" rather than "single" (those listing single generally lean monogamous). With an honest profile you'll have a better start on how to explain your situation and it'll be well received. My husband hasn't found a "one" but he's met some really nice ladies and remains optimistic.
Don't know many things to watch out for (the more experienced will hopefully chime in), just suggest to keep it real, be yourself, look to have fun, be respectful and keep the communication lines open with your wife and potential sweeties.
Best of luck! Welcome to Next.... and congrats for finding a way to keep the family together (happily, I hope) for your young son. What a gift!