Cut on Thumb

My boy sucks his left thumb, I don't think I've seen him suck his right. He has his front two bottom teeth; and his top teeth are going to cut through any day, so he's sucking a lot more. I just noticed a welty cut on his left thumb I'm certain is from his lower teeth, and it's making me nervous. I don't know how it's going to heal, or how I'm going to keep it from getting infected or something- it spends so much time in his mouth. Is there anything I can do?