I don’t know about administering herbs through breastmilk, but you could try making a tea bag of mints and Horehound (also a mint) and some eucalyptus, if you’ve got it, and stand in the shower with the baby, while the teabag hangs under the faucet. That would help ease his breathing. How old is this baby? I’m surprised he isn’t in the hospital –RSV in infants is kind of serious.
You can also put some essential oils in the medicine chamber of a humidifier.
If you’ve bundled some mullein and/or sage into smudge sticks, you can burn that in the room the baby is in.