Im going to take a guess that the biggest issue is that you havent met A. My boyfriend spends time with me and my children, or me my spouse and children, and has stayed over at my house with me and my children with spouse away for the night. BUT my boyfriend and spouse are on very good terms, and very comfortable with each other. My spouse can feel confidant that BF is a good person, good with children, and particularly likes ours. My first suggestion would be smoothing things with the relationship with A more before spending much time with the couples son.
keep on keeping on