My partners breath smells like onions after sucking my nipples. is this something i can fix?

His breath odor cannot possibly have anything to do with oral contact with your nipples or anywhere else on your body. It doesn't mean anything about you or your body (or its cleanliness).

What did his breath smell like before sucking on nipples? He may have a diet that contributes to the smell, or poor oral hygiene.

Most halitosis caused by sulfur producing bacteria lodged between teeth. Sounds like your partner may need to see Dentist.