Same boat over here, the book "raising lions" has help us so much, because the similar behaviors, my son also 5 , also bilingual and very..very smart I think you will get good result too. good luck!!!!
Thank you all for your thoughtful comments. I want to mention that we do read and seek advice, so have tried many of the best-practice methods regarding behavior. What we realized is that ALL the other parents we're friends with - these methods work perfectly for their children - but have no effect on our son....so that has been a point of frustration for us.
I should mention that he has sailed off the charts intelligence-wise. His reading, writing, memory, logic, math, music, spanish - his various preschool teachers all say they've pretty much seen nothing like it.
He processes and rationalizes situations around our behavior direction, and it almost unsettling how deeply he can talk about those rationalizations.
It's like he has the intelligence and brain of a 10 year old but the emotional intelligence of a young 5. Big imbalance. I wonder if this is the root of all our problems.
My advice is to maintain a consistent approach to this behavior and perhaps enhance the degree of structure you emply in managing the behavior. A very reliable approach can be found in Lynn Clark's book SOS Help for Parents (see www.sosprograms.com). As an aside, the two-minute time out is too brief. The time out should be from 5-10 minutes, with the time commencing only when he is seated in the time out chair and is quiet. If he makes any noise during the time out, start the time over. Use a digital cooking timer to track the time.
Sounds like he strive for the attention his younger siblings get (as they are younger and need more help), so best may be to show him that he is as much loved as his sister and brother, and give him time to be dependent and cared for as much as his siblings.
To add something to my original post about my 5 year old boy, there's also a lot of anger that sounds like frustration. I'll say "hey buddy, put your shoes where they belong" - and he screams "ahhhhhhh!", completely angered at the request - every time we ask him to do the smallest thing.
Worse, we'll say "hey, did you see this postcard from your Aunt?" He scream "I already saw it!!!! Ahhhhh!", somehow angered and frustrated that we even mentioned it.
Finally - he suddenly holds a huge sense of entitlement. He'll weave elaborate stories about why he should get ice cream or a new toy. We'll explain his behavior hasn't been such that he should be rewarded - then he says he'll just take money and buy it himself. He also says his behavior is all our fault because we don't give him toys, and unless we do, he'll act worse.
This is troubling because he's 5. He's smart, but he's somehow figured out how to rationalize and make demands.