Sick baby... HELP?!

So, when my baby was a month old he got a small cold. We took him to the doctor and they said just use saline and suck the boogers out. We've been doing that and I bought a hunidifier.

He's now 12 weeks old and STILL has a stopped up nose and now since thursday he's congested and coughing. No fever yet. I took him to the doctor yesterday and she said his lungs sounded fine and his nose looked fine and he was happy at the doctors office. And he was in a good mood for a little but this morning but he's been sleeping a LOT more than normal and he's went from eating 32 oz a day down to 23/24 oz a day..

She told me i could give him baby zarbees cough medicine. And I have been as he needs it. But I'm worried about his eating he will sometimes only take an ounce at a time

She said to either come back or go to the we if he runs fever won't take a bottle at all or stops having wet diapers. But with him having a stuffy nose since he was a month old and now coughing and congested on top I'm just concerned about it