5 week old who is congested

So I know infants have small nasal cavitys, and they always sound congested because they breath through their nose. But my 5 1/2 week old sounds super congested. He was with his cousins last night and the 9 month old and 5 month old had runny noses. Now my little guy has a little  ough but does it once every hour or so and sounds more nasally then usual. I exclusively breastfeed and had the humidifier going this morning. Is there anything else I should do?