Dear Amanda,
There is no doubt that children with sensory integration problems can also have behavioral difficulties associated with the SI deficits. It's not possible for me to imply that your son's oppositional behavior is a consequence of his sensory problem, though.
The most pertinent evaluations to conduct are a hearing assessment, an occupational therapy assessment, and a pediatric behavioral health evaluation. Information from all three can be integrated to better understand his difficulties and design appropriate intervention.