How do I get my ten month old to stop sucking on his bottem lip?

My son wanted to suck on a pacifier but could not keep it in his mouth. So he learned to suck on his bottem lip. He didn't do it much, just once or twice a day when he got upset. Then about a month ago when he was 9 months old he started sucking on it all the time. If he isn't eating or jabbering he is sucking on it, even in his sleep. It is starting to turn purple and I'm afraid he might end up drawing blood. I was just wondering if anyone had any ideas before I called the doctor to ask. I was hoping it was just a phase he would grow out of quick and I wouldn't have to take him to the doctor, but he doesn't seem to be letting up at all. If you have an idea please let me know.