I'd like to recommend getting an OT evaluation outside of the ed. system, too. It made a world of difference for my son when we did the Sensory Integration and Praxis Test with a qualified OT last year (gosh it's been a year!!!). We were better able to determine what my son's sensory needs were and how to work with him. Debbie could let you know if that is a good route for you to go too. She's the one who told us about it.
The meltdown after school is common. Usually these little guys just hold it together throughout the day then come home exhausted and sensory overwhelmed. I have my son's weighted blanket on hand for him to use. Most of the time he goes straight for it when he gets home. If we are in the car he uses the weighted lap pad.
I second Debbie about getting outside testing. We took our son to a OT for a SIPT evaluation. We took him to a neuropsychologist for a total educational and neurological evaluation. We took him to a neurologist for diagnosis.
Another good read for you is The Mislabeled Child by the Eides. That and the Misdiagnosis one Debbie recommended will give you a very good idea of what all a gifted child has to deal with.
Mind you, your sons scores are high. On the WISC IV the confidence interval is 6pts+/-. Given the areas of deficit is is quite conceivable your son's iq is higher than the test predicts. Also given that the test was most likely not done by a gifted specialist....well, like Debbie says, get the outside eval.
godspeed! Your son is lucky to have you for a mom, cuz you obviously are watching out for him!!! Go for it!!
