Nasty tasting cum 🤢

So my boyfriend is an amazing guy, I really love him soooo much but it’s just honestly been really hard looking him in the eyes and telling him that his cum tastes like battery acid. What could cause this???? His precum tastes good, like my cum, but his actual cum tastes completely different I have no idea what’s going on.

His penis looks normal.

He’s a very clean guy.

We’ve had a lot of unprotected sex previously so I went and got tested and I don’t have anything.

What could make his cum taste so bad?????