What Causes Smell From Head Of Penis?

Partner has rotten/rancid smell around head of penis. Is it possible that foreskin is not being pulled back all the way when he is bathing? We noticed it last night when penis was erect. It was a very big turn-off and one I am having a 'hard' time getting over. I feel like I am going to want to check him out now before oral sex...this may be upsetting to him, but this has freaked me out!