After spending way too much time reading about various meds, I have concluded that any of them might work, might do nothing, or might have negative side effects including making the thing you are taking them for worse.
It is such a crapshoot. That's why I was so excited about the DNA thing I posted in another of your threads.
I hope the new combination helps your DS. It's no wonder he's resistant to going to school when the year has been so overwhelming for him so far. I know you've said he has a therapist--I hope that person can help. We don't have the "perfectionism" piece in DS, but I have a super anxious gifted perfectionistic DD and she is WAY more difficult to manage than her brother (ironically). Is your DS open to things like affirmations, breathing exercises, etc.? It would be helpful for him to internalize that it's okay for him to struggle, it won't always be this way, one day at a time, etc. Anxiety is really treatable, but not a quick or easy road.
A scientific explanation to your DS about the neurology of anxiety might be helpful, too, since he is a logic oriented type. Beginning to identify--"oh, my limbic system is hyperaroused because the fight/flight/freeze mechanism has been activated" might help him a little. I know that sounds goofy, but it is really helpful for some people to understand what's happening to their actual physical body, and can reduce some of the anxiety that results from anxiety. If that makes sense.
There are probably good videos on YouTube or explanations of the anatomy of anxiety/panic--wonder if any of that would help. DS' therapist is educating him about "helpful vs. unhelpful worries" as part of his CBT. Helpful worries lead to positive behavioral change while unhelpful ones lead to avoidance, withdrawal, meltdown, etc. using this framework.
I think with brainy kids, sometimes you gain better access via cognition and you might get more buy-in from your DS if he understands the physiology of his experiences.