Take him to a dentist. Do you ever remember white lines up there? My 2 year old has these, she's got other dental problems also (i also brush her teeth every night before bed and durring the day depending on what she's eaten that day, she's a picky eater so she doesn't eat much junk and sugar). She had a chip out of the back of her front tooth that turned into a cavity, not she has a chip (noticable) out of the tooth and a molar that looks pretty bad. We are now waiting for an apt at the childrens hospital because she will have to be put out in order for the dentist to work on her. I can't really tell you what is causing your sons lines, but i know the dentist i took my daughter to told me to really watch the white lines to make sure they don't turn yellow because that is a sign of decay...i would take him in asap, you want to catch any decay before it gets worse.