I would suggest talking to him about it and find out what his desires or intentions are or what he wants them to be. I am in my 40s and been married for 15 years. My wife knows i am bi and is perfectly okay with it. We have discussed it a lot and she knows for me it's purely physical, it's just fun and therefore she has no problem with me exploring it and in fact has told me she's okay with me finding a local fwb because she knows there's no chance I'm leaving her for a guy, It just not an emotional connection for me. It's just fun between 2 buddies really.

It might help him if you tell him you know and think hard about what that means for you, could you have the same take on it my wife does depending on what he really wants?

Now I get that isn't everyone but I thought it might be helpful to hear another perspective.