It is likely that he has an allergic reaction to something that is irritating his nose or coming down with a cold
The other thing to watch for in children, daft as it may sound, is threadworms. Ask him if he gets an itchy bottom and especially a night time. If you help to clean his bottom just look at the stools to check for any threadworms. Threadworms, look like very thin pieces of white thread.
Best wishes.
Hello, it is quite possible it could just be a habit that began when he had a cold, since you don't see a rash or sore. If he seems to keep being bothered by it though, it could be he has allergies, that you are not aware of. Bring up the subject at his next doctor's visit. Best wishes