Persistent thoughts are not that uncommon with anxiety, although it can feel very distressing. My DD has struggled with this at times. As her therapist explained, these are thoughts that everyone has from time to time. The difference is that they tend to get "stuck" in the minds of some people with anxiety.
Medication may help, as can just working through the thoughts with a therapist.
But know that it doesn't mean your DS is highly unusual or that this is a harbinger of dire outcomes.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)