If he's not feeding enough to stay hydrated, I would take him to the GP, personally...he's still v little (esp being prem).
Is he keeping down what he's drinking? Can you get him to drink some cooled boiled water, if he's not coping with milk?
Humidifiers are great, and steam does help, for bunged noses. I find Olbas junior smells better than Karvol, but its much of a muchness.
That said, my dd2 had RSV bronchiolitis at 4 weeks, and it did look like a cold, bunged up, and difficulty feeding, to start with - if he starts throwing up feeds, struggling to breathe (watch his ribs, check if they suck in or not - and his throat, too) or seems dehydrated, I would take him for a check up asap.
Probably bunged up nose, not trying to scare you, just wanted to mention it in case