Good you have set a meeting for clear thoughtful discussions. The outcome depends on several factors but mostly on a calm loving discussion with lots of listening. His feelings of security and comfort in your relationship and his is a big one.
Does he enjoy knowing of your romantic encounters? If so that could be a double bonus. If not then it could become a give and take discussion. This is a big step for all 3 (or more) of you. My advice is to listen to his reaction before you push too hard to sell the idea. He may surprise you and want to simple give you this added benefit (compersion) the act of receiving pleasure from giving your significant other the freedom of pleasure with another.
Ooop, gotta run... more another time.
Love to all