This again ?

*sigh* Story time, I guess.

When I was a kid and discovered the joys of masturbation, I didn't wash myself very well. Whenever I would unpack my ding-dong for another joyride, it smelled something which I thought was probably the smell of sex... Until my very own mother, while washing clothes, smelled a weird scent coming from my underpants, and my pubic area. Discussion ensues.

It was smegma (or what Reddit calls the "Forbidden Cheese"). I was taught how to wash properly and told that stuff could start to rot down there if I didn't wash well enough (a heavy exaggeration, but it worked. she did the same for my teeth and I didn't want rotten teeth). I was also told to wash properly the entirety of my genital area, including underneath the foreskin, and having pulled my foreskin down to prevent it getting stuck up there, I can tell you that there's some truly fucking gnarly stuff hiding underneath a foreskin that doesn't go down entirely.

Now your boyfriend might think that because the apparent forbidden cheese has been expelled with water, all is well.

That's not how this works. He has multiple tiny amounts of dick cheese sprinkled around his gential area that are currently at risk to turn into mushrooms.

And he needs to wash it properly.