Not supposed to have bad breath this young though.. Did you clean his mouth thoroughly? Maybe you can try getting the silicone finger toothbrush. It’s very soft, won’t hurt baby. But at the same time, it can really clean away all the milk residues on baby’s tongue.
https://welovesupermom.com/products/sft-5610-108-kidsme-silicon-finger-toothbrush-and-gum-massager
Read more