It can be like drawing teeth trying to get kids to open up! He probably has a lot of hurt buried and is scared to even look at it, let alone share it.
Can I suggest going out for a milkshake or something with him, some regular 'special time' just the two of you. Don't batter into the subject straight away, wait until he's relaxed and then raise it and see if you can get past 'I hate school'. Mostly, let him know that you love him and you want to help him. Just gently see if you can start to tease out the problem and get to how he's feeling.
I know my son's patterns very well by now - if something's upset him he'll act up, be a total pain. I then have to grit my teeth and gently ask what's wrong. Eventually he'll get to 'it's boring', then I know I'm on track. I keep going with more patient listening, not getting angry (at least not showing it), staying supportive and encouraging, and eventually I'll get to the problem. Once he's let me know what it is, maybe had a bit of a cry or a rant, he's generally about 80% better and back to my lovely boy again. It's all about emotional regulation.
It is bloody hard work, I won't lie, but it's a thousand times better than the massive, endless fights and tantrums we used to have. Just a case of building trust and allowing him to have a space where he can open up and tell me what the problem is.
Special needs can be in varying degrees. My son really struggled in one class because it was chaotic, very messy and there was masses of stuff stuck up on the walls. (Some kids thrive on this kind of stimulus; some get very anxious). We asked for him to be moved seats so he wasn't looking at the worst wall, this did help a bit.
Also finally getting a diagnosis of dyslexia really helped. Instead of being shouted at for not trying, he was given extra help. This has turned things around.
Sounds like you're doing you're best, OP. Sending you best wishes, hope things improve for you soon.