My teen son has an anxiety disorder. when he feels a pain, he thinks it's cancer. recently, he's been having abdominal pain on the left side (sometimes other side), he says it got worse after he got anxious over it. is this "colon cancer", or anxiety?

Left sided abdominal discomfort is often due to constipation. A teenager is unlikely to have colon cancer but getting this treated if it is constipation would be helpful. So, he should be seen by his doctor if this discomfort continues, especially if he is constipated. Health anxiety can be treated by psychiatry so that this does not continue to be an issue for him throughout his life. Good luck!