Yes, I think you should bring him for an evaluation. It will help to discern the nature of his worry. One thing the clinician will focus on is whether your son is actually experiencing impulses to hurt the dog (and under what conditions those impulses occur) vs having a worry (but not an impulse) to harm the dog. It goes without saying that he should not be left alone with the dog. One of the possibilities is that your son is displaying a form of Obsessive-Compulsive Disorder (there is a high correlation bewteen TD and OCD).