Son's Lip sucking

Just wondering if anyone else has come across this. My son is nearly 5 years old and since he was about 2 years he's had a habit of sucking his bottom lip. He does this during the day and at night time.When he started school (he was just turned 4 the month before) we started mentioning the lip sucking to him and trying to encourage him to stop, which did reduce it but now he's back to lip sucking all the time now. He seems quite a happy and confident little boy sometimes a little shy, but I honestly think he doesn't realise he's doing it. His teacher has mentioned it and wondered what we can do to help him stop. His bottom lip gets really swollen and in winter really chapped. Has anyone heard of this before??

I've heard this can be either a comfort thing, kind of a daydreaming and sucking lip or, alternatively, an anxious thing. Do you notice he's doing it at particular times or in particular situations? It may just be something he enjoys - like thumb sucking in other children.

I think to gently encourage him to stop if it's causing a problem with his lip but I wouldn't make too big a deal of it. He'll probably just stop it over time.

Thanks for your reply. Think it was a comfort thing but now he does it constantly without realising it. I've said in the holidays we'll maybe try a sticker chart to try to encourage him to stop.