So you are in love. That's fine. Feel what you feel when you feel it. Nobody can help how they feel.
But we CAN help how we choose -- to just REACT or act with intention.
In this case? I would act with intention and make it clear that this is a friendship only and keep it in the bucket. Keep (and enjoy) your love feelings to yourself. There's nothing wrong with enjoying love from afar with no return. Crushies are crushies.
BUT... Don't feed a lost cause there by fueling his ill-conceived fantasy. Keep it in the friend bucket.
THEY are not open and they seem rocky. You say you don't want to get in the middle and be like some homewrecker.
So you can be a supportive friend at best only -- encourage him to communicate with wife and sort out whatever rocky they got (and you don't need to know about it.)
Tell him you're not interested in that kind of relationship with him, you want a friendship. If he insists, tell him he's not being respectful of you. If he insists again, stop spending time with him. It doesn't really matter why you don't want a relationship with him. If you say you don't, he should respect that.
Yup. Keep it honorable and keep it clean. And if you can't? Do the honorable thing and cut it loose.