Hello and welcome to the forums!
Your openness and willingness to explore poly is very refreshing, especially if it is not something you had considered in the past.
I do have a question if I may ask. You did not mention his wife and if you have met her and all are comfortable with the relationships. The only reason I ask, is my wife and I always try to ensure the her boyfriend (she is non-partnered at the moment) that we get along well also and are comfortable in each other presence when all three of us are together.
But, sounds like it is working well and no issues. Good luck for continued success and hope to hear updates as it goes along.