Anyone transtion LO from bottle to sippy before 12 mos?

sescms2004

DS has always been an 'aggressive eater'.  We've had to move him up to the next stage nipple each time, usually 1-2 mos early.  LO's are 10.5 mos and both on Stage 4 nipples.  DS is struggling to eat, just like he did every time it was time to move up in nipple stages, but I don't know where we go from here?  He is taking 20 minutes or more just to take 2-3 oz and then gets tired and doesn't want to finish.  I've tried offering his formula in a sippy and he refuses it (but he will drink water from it). 

Did anyone else go through this?  DH thought maybe we could try the Y cut nipples for thicker formulas, but I don't know if that's a good solution?  Any advice appreciated!