Hi Michele
You are concerned as your 5 year old has started dry sniffing on a regular basis.
Its most likely that this is due to habit, rather than a physical condition.
A good approach to this kind of behaviour is often to ignore the behaviour, and try to avoid drawing attention to it.
If you do notice him doing it then its a good idea to try distracting him ( ideally without mentioning the sniffing).
Hope this helps
Tracey