I don't want to kiss my boyfriend in the morning if we haven't brushed our teeth
Why do you not want to kiss him then?
His breath smells bad and it is gross.
As long as you share with him your reasoning, I think that's okay. Make sure he knows it's because you feel gross in the morning, though, so he doesn't think you are rejecting him. However, it's always okay to say no to physical contact, and you should have your wishes respected.
I told him and he's making fun of it.
That's a bit juvenile but it's better than him getting offended, I guess!  What's good is you stood up for yourself and suggested a change in an empathetic way.