I was going to recommend the Nuby, but I see that you've tried it. Was it the soft spouted one? I found this one to be most like a bottle nipple and we were very successful with it. I used it as a bottle to sippy transition. (Like another Mom has suggested, make the bottles disappear so that DS can't find anymore... that may help.) You could also go back to a beginning nipple (Avent called them 1's). This may frustrate him and make him want to use a sippy so that he doesn't have to work so hard to get his drink... I know that's mean, but it might work!
I'm sure you've also tried mixing formula 1/2 and 1/2 with milk, or even start with more formula, less milk and increase the milk little by little. I thought the flavored milk might be a good suggestion, but then you might get yourself into a situation where DS always expects that, too, and that isn't a good habit to start.
Sorry I couldn't be of more help. Good luck.
T. B.
Mom to Katelyn, 19 months