My son is ADHD/ODD, diagnosed at age 9. Defiance can come from ODD or due to meds, so it’s difficult without knowing more background to say. I will say when my son was 6 we also thought he was SPD because he would chew on his shirt, pick at his skin, he liked things loud. Hindsight 20/20 I think he did these things because he was also anxious about not being able to sit still, and he did not want to get in trouble in class. He is currently on a non-stimulant medication, because our Pediatrician was concerned a stimulant may cause more aggression than we already experienced (have you discussed this possibility with our Ped?). The medication helps, but it doesn’t fix everything. A lot of his aggression has dissipated, but we have our moments. But, to really know for sure what your son is dealing with you should seek a counselor.