My four year old boy has told me that his mouth is hurting, and upon examination, it turns out he has some kind of white hole-like circle on his lower gums beneath his incisor. I'm not sure if this is a canker sore or something else. He has had a canker sore before that looked different (big and puffy, not hole-like), and it went away on its own. I have had him brushing his teeth and rinsing with salt water, but it's not going away. Do I just wait it out, or should I have him checked out by a dentist to see if it's something else?