Once a habit like this starts, it can be hard to break. I used to have a child that chewed on her lips and made them so chapped it was painful to look at them.

You can try giving him something to chew on...a wet washcloth will do the trick... keeping him distracted and happy with other things will help. He probably doesn't even realize he is doing it most of the time now, so you will need to constantly remind him not to do it.

What happens is that the external cells of our body are filled with endorphins, and a little chew on the lips is all that's needed to release them. So, the release of these endorphins elevates mood and becomes addictive, even if we don't realize what we are doing.