I have a dcb much like yours. Except he is 7! I have never seen the likes of this child when it comes to tantrums.
I tried the talking it out business for a while. Didn't work, he loved the extra attention. So he gets the "Too bad you don't want to join us now. When you are done crying, you can." Then we hightail it out of the room. The worst time was a 20 minute tantrum because I wouldn't tell him the answwer to a homework problem.
Now that I ignore it, he has gotten better for the most part. There are still bad days once in a while, especially when he is overly tired or overstimulated from school. But we have gone from once a day to 2x a month. I also make sure to give him extra attention when he is being his sweet self and thank him for being a good example to the little ones.
(And yes, I have talked to mom about an eval, which I think he desperately needs, but it hasn't happened as of yet
)