I recently happened to see an episode of Super Nanny where the parents were going through the exact same thing you are. The little boy (he was 3 or 4) didn't want to give up his bottle. The way around it was the nanny told the little boy what a big boy he was and that he didn't need the bottle anymore. The whole family (Mom and Dad were in on the plan and they did it together as a family). They worked their way up to the little boy throwing away the bottles that he used to use in the garbage can (that was the first major step) and the little boy had to throw them away on his own (Mom helped aim them into the garbage - they were at a beach in Hawaii and it was a city trash can along the beach - not sure if throwing it into a city can makes a difference??). Anyway, the next major thing was that the father was in the ocean (ankle deep) and was like, "what's this??" - they put a sippee cup into the ocean on a string and made it seem like the Dad found it - they made this HUGE deal that it was the little boy's new "special cup" - his very own to use and not his brother's or anyone else's. You could do something like that (maybe not on the beach in Hawaii, but come up with your own creative plan on how the "special cup" arrives for your little boy). It totally worked too - the little boy never went back to the bottle after that.
Another thing it could possibly be is that your little boy wants to be like a baby - especially with two other little ones in your home. My mom told me that when I was little, I wanted off the bottle and wanted to use a cup like my older sister. I got off the bottle early, but then when I saw a baby with a bottle, it hit home and I really felt like I was missing out. It just looked like it was more fun than a cup, I guess.
Anyway, good luck to you!
S. (Mom of Jackson - 3 years old Sept. 8th)