I just don't know....

If hes uncircumcise it could be a lack of regular cleaning in his part.
Uncircumcised penis gets dirty alot more quicker because of the closed space making it easier for bacterial build up.

The longer he doesn't clean it the more it stench up. Especially when he ejactculates and that two smell combine, It creates a whole new horrid smell. I fucking kidd you not. They have it named in my country. My own mother even experience this. I did not need to know...

I was not circumcise in birth but in my teens. So i know the two sides of this coin.

Suggest a steamy, soapy shower together.

Just tell him

I really think that it's an STI.

Is he uncircumcised? It is not uncommon for uncut men to form smegma underneath the foreskin. Bacteria and smegma can build up if you don't properly clean underneath the foreskin. Buildup will cause a smell. If he is circumcised, it's a different matter. It could be dried semen on his pubic hair covered testicles. An infection. He might want to see a doctor.

It could be a ritual before either jacking him off of having sex to wash him down beforehand. Obviously, he doesn't take care that he should. You on the other hand can make sure it doesn't stink or even have the bad sex smell that some guys have. Good luck.

It shouldn't smell unless he doesn't wash it.

Could be an STD or he's just one dirty fuck. It's not that hard for a man to wash his dick.

I don't know about an uncircumcised penis, but a circumcised penis should not smell, beyond the person's normal odor. I'm guessing your bf just doesn't wash himself enough. Perhaps you could incorporate washing his penis into your sex games and that might help.
I don't think most guys would take too kindly to, "Honey, I love you, but your penis smells. Please wash it." We are all a bit overly sensitive about THAT organ, so you are caught between the rock and a hard on.
Good luck.