My guy tends to attack his sister when he wants attention from us, or wants her out of his way. We've had to really watch to figure out some of them - I completely know what you mean when it seems out of the blue (because most of them seem off the wall for the longest time, and some you just can't ever figure out)!
My son is very protective over toys as well. Sharing is an absolute nightmare. He thinks everything is his, and everything should be where he put it and everyone should leave it alone. I have to sit in between them many, many times just for my daughter to be able to play.
School has been a blessing - the routine, being around other kids, me getting a break - it made a HUGE difference.
As far as the never having issues before... we've been there so many times as well. I don't know what happens, but like a light switch, something he loves becomes something he can't tolerate, and vice versa.
I did the whole documenting thing as well with him, and most of it was sensory related. My son understands that if he shows me whats wrong I will try and fix it, he will just randomly walk up and pinch his brother and that part I cant figure out. I dont know if its because his brother isnt playing with him or its the toy his little brother has. He is already enrolled in school and starts in september, they have a plan written out for him and will be working on language, and sensory. one morning he actually wet through his diaper and I had no choice but to change his shorts, he got upset, refused to eat breakfast and was in a violent rage for 2 1/2 hours because I wouldnt put the same shorts back on him. Last night my husband had to literally hold him down so I could change his diaper, he has never had issues with diaper change. It just seems like his anger is getting worse. he acts out to everything now.
It's a possibility - I mean, there are kids who do have bipolar.
I have a son with autism who has aggressive outbursts. I also know a lot of people with kids with autism disorders that are violent. It sounds a lot like what you're describing.
I think what you should ask EDIS about is a behavioral plan and getting set up with therapies.
Remember his verbal is delayed, which is frustrating to him. Imagine not being able to tell people what you wanted, especially when you know what you want (and we all know that toddlers/young children KNOW what they want)... I can only imagine how frustrating and upsetting that would be!
For a long time, I thought my son's tantrums were random, but I started documenting what was happening before/during/after the tantrum. Is he getting something out of them? For instance, my son would hit his sister at bath time because he knew we'd get him out (he hated the water more than he hated being in trouble). He also hated switching clothes - sensory issues... he didn't like the feel I guess of having his clothes changed.
Good luck :) I hope the psychologist and the EDIS people can help you. Early intervention wasn't helpful for us, but I got involved in every local resource I could to learn more about autism.
Honestly that's hard to say as bipolar is extremely rare at that age and autism can cause these kind of destructive behaviors. You could mention it to the psychologist though if you are concerned though it generally expresses itself at a later age but only a psychiatrist would be able to provide a diagnosis. Cognitive behavioral therapy can be helpful on autism in helping retrain children as regards behavioral patterns and as it is a developmental disability unless a psychiatric disability or physical disability (such as epilepsy which can sometimes accompany autism) as well exists autism is generally not very responsive to medication and as for how well behavioral pattens can be controlled it would dependent on the severity of the autism though only a psychiatrist could explain this in specific detail as regards to your son.