Partner has bad breath 😷

I feel bad writing this because it’s a secret I’ve been keeping from my boyfriend for months, but I can’t lie anymore: HIS BREATH REAKS. It smells like mothballs and it wafts up my nostrils anytime he talks to me. It’s becoming a real turn off and I feel really bad thinking it and I don’t know how to break it to him (obviously I wouldn’t put it the way I just did). Have you ever had to tell a loved one their breath stank? How did you do it in a way that didn’t hurt their feelings too much? Also why does it smell so strongly of mothballs?!?!