Thumb sucking

My son sucked his thumb until he was 4 months old, which I didn't have a problem with because I was pretty sure he would grow out of it. Now at 10 months he's started sucking it again. He'll go from chewing on it to sucking it randomly but he still takes a pacifier before he goes to sleep. Tell me if your babies grew out of it on their own or if you had to break the habit yourself