My 11 month old also has Pierre Robin. We feed him with the Mead-johnson cleft palate nurser bottle (no n/g tube, luckily). He started eating baby food at 6 months, and while he eats 3 meals a day now of baby food, I am still feeding him over 30 ounces a day of formula/breastmilk by bottle. He just doesn't have enough motor skills yet to drink from a cup (so I understand your issue with the sippy cup pouring out), but he is practicing with cups. Even with his palate repair (done a couple of weeks ago), we are still using the mead-johnson bottle.
As others have replied, they still need lots of volume of formula/breastmilk even past 12 months old...
Most importantly, if you haven't already found out about it, there is a good, active yahoo group for Pierre Robin families.
Good luck!