Good advice on the anxiety. With self-talk that negative, you may want to consider a more structured approach to the suggested techniques. For DIY, there's some good websites and workbooks, for instance,
www.anxietybc.com which has practical stuff for both adult and youth. There's been a couple of previous threads with a lot of book recommendations too (if a much better searcher than I could dig them up - hint, hint Indigo). Or cognitive behavioral therapy (CBT) with a specialist aims to help learn how to identify unrealistic thoughts, assess them, and turn them into less catastrophic or negative views.
I have to agree with others that if he hates school, then school is not meeting his needs, no matter how loving or supportive it may be. Really kind, well-meaning teachers doing the wrong thing can cause a LOT of damage, though they would be horrified to realize it. And with 2E kids, doing the wrong thing is really easy. He may be spending his day frustrated by work that is way too easy, while simultaneously spiralling up the anxiety because simple tasks that he is expected to do aren't getting easier for him - but are getting easier for everyone else - and he's working harder and harder to keep up from drowning as task complexity rises. Which it starts doing fast around grade 3. With my own and many other 2E kids I've seen, anxiety tends to be the biggest symptom, visible long before clear achievement problems manifest.
So knowing exactly what you are dealing with in your 2E can be critical, but figuring that out can be quite hard. Gifted kids are really good at using heroic compensation measures to maintain grade-average function for waaaaay too long. You need a specialist who really understands how they can hide their weaknesses and who digs deep, to make sure you are not missing problems. For instance, you said dyslexia was ruled out. When you have your meeting, make sure that this conclusion was reached through direct measures of the underlying cognitive processes of reading (e.g. direct phonological measures) and not inferred from average reading achievement scores (which gifted kids can fake frighteningly well. Trust me on this one.)
My DS13 has what looks like some pretty extreme inattentive ADHD (his younger sister is comparatively mild). We have not yet tried medication, but will be doing so with DS over the next month. DD11 is reasonably functional at school as long as the work is appropriately taught and interesting. DS, however, is losing his mind in frustration at his own inability to keep himself on task and get his work done in a remotely reasonable amount of time, even when that work is very interesting.
Note that anxiety cannot cause inattentive ADHD - but anxiety can definitely destroy concentration and therefore look like an attention problem. Anxiety tends to be a big red warning flag that learning needs - for both strengths and weaknesses - are not being met, and major change is needed. It may be that his current school is able to learn and change enough to meet his needs, or he may need to move to an environment that better understands and has experience with kids like him.
