I can completely relate to the situation your husband is in... mono husband to poly wife, she the only woman I have ever been intimate with, me content with it being that way, me not wanting her to want other lovers.
Your latest response indicates you have taken a step back to look at the big picture which I applaud you for.
A couple of things from your OP I think are important:
1) Your husband grew up in a different culture. I would think those cultural differences matter and need to be taken into account.
2) Your "other man" has a girlfriend and a child living at home (not clear if it is his or the gf's or theirs). What do they know about the situation? It seems there several people who would be affected by what decisions are made.
What EugenePoet said is so true... I have been "ambushed" by emotions I have never had to face before.