Good luck with getting some intervention/assessment through your brother. In the meantime, I would make your own observations and adjust your parenting/management of your son with a working hypothesis of Autism Spectrum Disorders (ASD). However, use your own instincts, try this and that to see how it works for him. USe what he's fascinated with as a tool.
A very useful reward I recommend you use - this is what difficult child 3's psychologist recommended for us, it was brilliant - tell him that if he can NOT have a meltdown for a whole day (at home - don't inflict school problems on the home front, you weren't there at school with him) then YOU will play on the PS2 with him for half an hour. We would do it the next day, because the worst meltdown time is when they're tired and heading for bed.
Our guidelines - if difficult child 3 had enough of a meltdown to have to be sent to his room, then no gaming for that day.
What this does - he has a HUGE incentive to really try to hold it together at home. And the payoff is enormous, as well as helping him with social skills. difficult child 3 & I would play a game like Mario Party, which works a bit like a board game. He would let me practice before a mini-game because I'm really not as skilled as he is. We would have a lovely play together.
Sometimes we wouldn't have time on a particular day so we would have to hold off the game until another day. Game time, once earned, cannot be taken away. Not even the worst tantrum of all can eliminate the game together time earned the day before, for example. However, you might be able to say, "I don't want to play right now while I am angry, but I will play the promised game time with you later." Even if you have a chart on the wall where you note down earned game time, so HE knows that it won't be forgotten... although I found difficult child 3 could hold the information in his head, on how much game time had been accrued.
And a warning - this may not work for you RIGHT NOW as well as it did for us, because I suspect a big part of difficult child's anger right now is, he knows there is something wrong and he is feeling very angry (a lot of it with himself; and a lot with everybody else for not following HIS internal rules which he's worked out by observation, so far) as well as frustrated. We found this happened with difficult child 1 when he was 6 - he got a diagnosis of ADHD at about then which helped him accept that it wasn't his fault, although ti did mean he had to work harder at some things. And it happened with difficult child 3 when he was 8 - he would come home from school and ask, "Why am I different?" He'd been diagnosed at 3 but simply hadn't had the language capacity at the time to understand it. It really did take until he was 8 before he 'got it'.
Working with your child's issues like this can give him a push start in the directions he needs to go, even before you get a diagnosis. If later on you get a different label, you still have done something which works for your son because you've used his own issues to help him learn some measure of self-control. But you might find you need to tell him - "Honey, it's not your fault, but it IS your problem. Your brain is working a different way to a lot of other kids, which could be why you find others so frustrating at times. And they don't always understand the things that are important to you, and they will find YOU frustrating. We're trying to find out just what the situation is, so we can help your brain learn in the way it needs to learn."
Don't describe it as a disability if you can avoid it - you do not want him thinking he is flawed or inferior. Similarly, try to avoid boosting his self-esteem to the point where he feels he is superior - other kids will really resent him if he comes across as, "I'm smarter than you," even if he is.
We told our boys, "Some things are much easier for you, other things are much harder. You need to really concentrate to find a way to do the harder things, but enjoy being able to do the easy things because not everybody can. It balances out."
Once he can understand that it's not his fault, his behaviour may improve. What is then left is the normal frustration of trying to mesh his way of thinking, to everyone else's way of communicating. This is something we all have to do, but for a lot of difficult children it is a huge task. It will tire him mentally and physically. On a day when he's really had to use his brain to make sense of the world, or if he's had a lot of new things to deal with, he will be much more tired and likely to be much more irritable. I developed a cue for difficult child 3 so I could say, "Honey, you're getting a bit loud and sound like you're shouting at me. I think you've had a very tiring, exhausting day. How about we plan for an early night for you? Or would you like to have a little quiet time now, to help you feel a bit better? How about yo go to a quiet place, maybe your room, and read a book for a little while? Or listen to some music?"
Music can really help. There is a mathematical logic to music. Expose him to it, get a feel for what he's attracted to and what he rejects. Some good classical composers to begin with - Handel, Mozart, Bach (those old Moog synthesiser tracks are great) and Vivaldi ("The Seasons"). The soundtracks from the "Fantasia" films are also good, especially if they've watched them and enjoy them.
By not getting cross every time we were able to short-circuit a lot of difficult child 3's anger, much of which, it turned out, was triggered by negative social interaction at school.
Good luck. Let us know how you get on.