Hi . I hope you don't mind me jumping and posting but it's been suggested I might be able to offer a persepctive that can help. MummytoSteven knows me, I have two young lads, the eldest of which is diagnosed ASD (he's 3.5) and I have a diagnosis of Aspergers. I only got my official diagnosis last November, but that was mainly because I had no clue others saw me as different and thought my struggles were just me. So it was a bit of a shock when my mum admitted to me herself, my dad, teachers and paediatricians had all noticed I was different but my mum had deliberately downplayed things as she didn't want me in a special school.
Anyway, this is PURELY from my own perspective, but these are some things that might help.
1: If he's like me he'll find it nigh on impossible to talk about his feelings and why he's upset. It's not a case of being stubborn, or shy, he genuinely won't be able to think of the words. Or (like me) he may hanve the words in his mind but be unable to get them out, sort of like a locked door in his head. So if he can write sit him down, on his own and tell him to keep a diary, or write a letter, detailing things that are upsetting him. If he can't write then tell him to draw.
2: It sounds like he is getting overloaded, probably lots of things building up in the day. I get very stressed in crowded situations, because there is too much visual and auditory information. When I'm really stressed (or sometimes just because) I sort of switch off and literally cannot understand what the person is saying. It is very difficult to explain when I can hold an intelligent conversation on the Roman Emperor Domitian and then not be able to understand "can you pass the salt" a few minutes later. I also cannot concentrate on looking at too many things. If I'm in a familiar place, or one that is quiet my eyes automatically just look at a small section of stuff, the double yellow lines, or a bit of brick wall, for example. If I'm in a strange environment, or I need to look for something I can't cope with the additional visual information so I go very tense and stressed and start snapping or saying "fine!" I internalise my difficulties, your son appears to externalise them. If he is given something like a book, or some quiet music, or is able to walk off somewhere quiet, that may be the help he needs to settle.
I need time and space on my own, especially after dealing with large groups of people. Just a few minutes sometimes can be enough to make me feel better. Letting your son stay under his desk for an agreed five minutes at the start and end of each lesson may help him. I presume it's the feeling of being a bit enclosed and apart that he likes.
I had a terrible temper as a child and adolescent, really bad, although what I do want to make clear (because this will almost certainly apply to your son as well) is that nothing I did was ever for attention, in the sense that I wanted to get my own way. I might have sometimes been trying to tell things that I couldn't articulate (though I still can't be sure on that, I'm just surmising based on behaviours following incidents) but I never wanted to draw attention to myself.