Vinnyc is right. You're not washing it thoroughly enough. Sometimes when I use the bathroom after our 15yo son there's a strong smell left behind by him. It isn't a fart or shit or armpit smell, it's the small of stale cock left behind after he's jacked off. I know he does his jacking off in there because I had to ask him to make a better job of catching it in a tissue after finding spunk on the wall and radiator. I also have a pretty good idea that he doesn't wash under his foreskin too well because when he was young enough that we were still supervising his washing, he was very reluctant to pull his foreskin back and touch the glans as it was so sensetive. My wife and I didn't want to make a fuss about this. We simply suggested that he should try to do it for hygene reasons, and left him to his own devices hoping that he'd get the hang of it in his own time. Needless to say, by the time he was 10 or so we'd stopped being in the bathroom with him. But, however long he spends lounging in the bath, he never seems to make the water very soapy. We've had to plead with him to remember to lather up his armpits because he was still smelly after bathing. He's got the hang of that, but I suspect that he still isn't washing properly under his foreskin. I have been thinking about having a father/son chat about it with him, but it's a matter of choosing a good moment. Not easy.

Under the foreskin is a lovely warm dark moist environment in which bacteria are very happy. All they need is some food in the form of dead skin cells and away they go breeding like fury. What's happenning is that you are not removing all traces of the greasey film of dead skin cells (smegma) and the remaining bacteria just breed again. Within an hour of washing it's smelly again. If you wash it thoroughly and frequently enough the bacteria population will not get a chance to win the battle.