I'm with SRL. Can't make suggestions. Often very young kids are hard to diagnose and the diagnosis could change--I medicated my son very young and, in my situation, I'm sorry I did. It turned out he'd been diagnosed wrong a few times. Not saying YOU should not medicate, but this is what happened to us. Son is fourteen now and doing best NOT on medications.
I would go the non-medication route first, if I had it to do over again, and get a complete evaluation by a neuropsychologist, then get a second opinion before I rushed into medicating. Did your child get evaluated at the public school for early resources?
Are there psychiatric or neurological problems in the family tree? Substance abuse? How is your child's speech, motor skills, eye contact, interaction with peers?
Again, it's an individual choice what to do about medications, with no right or wrong answer. You best know your child. But if he didn't have a complete, long battery of tests, if it were me with hindsight, I'd do that first. ODD *rarely* stands alone--I'd bet every child here has ODD behavior and meets the criteria for ODD, but ODD is usually secondary to something else...