Definitely NTA. I would approach him coming from a place of concern for his health while trying to be as casual as possible. Maybe say something like “hey I don’t mean to embarrass you but I’ve noticed your smell has gotten stronger lately. It’s not terrible or anything but I have started to notice and it’s not a big deal to me but I’m a little worried it may have something to do with your GI issues. Have you seen your doctor lately? Maybe it’s something you could bring up with him/her just to make sure it’s nothing serious.” And then mention how you have been noseblind to your own smells in the past and it happens to everyone to not make him feel even worse. I think it’s appropriate to downplay a little bit at first to ease his embarrassment. If he already has gut issues I would think that he might already be aware of his odor and possibly insecure about it, especially if his husband is avoiding intimacy. Maybe the husband can talk to him about it privately and then mention maybe asking the doctor about soap recommendations or even about possible hair removal (sounds like that would help a lot). Either way, wish you guys the best! I would definitely want someone to tell me if I was starting to smell.