Embarrassing subject for boyfriend

Hi everyone, My boyfriend & I have been going out over a year but I have found it difficult to approach this subject with him and when I did, he didn't respond well. The question is, how do I say it so that it can be changed without hurting him? The problem is, sometimes (not always!) I feel that my boyfriend doesn't clean his bottom part properly, as I can sometimes smell an unpleasant smell and think, but not certain, that I have seen a mark on our friend's couch. I drunkenly spoke to him about my concerns one night and I could tell he didn't take it very well. Now I know no-one would respond well to saying that they noticed something like this, but I thought it would be better myself saying it to him instead of an outsider, who may have noticed. When I spotted a mark on his underwear he also said that this is due to flatulence. I've heard it's more difficult for guys to clean their bums, due to hair etc, but what could I do to encourage him to ensure his area is thoroughly clean without hurting his feelings? We spoke about it that one occasion but I like to reproach the subject.

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