my son would stay latched for and hour at atime and would normally fall alseep (still sucking) but if he stayed awake he would get a milk drunk and just kinda smily lazily (wicked cute btw) the pain you are experiensing , is it from a bad latch? that might keep LO from staying on long enough to get the hindmilk (which fills em up) you mom means well, but the reason you "dont have enough milk" is because of the formula. Also, withthe throwing up, is he maybe allergic/sensitive to the cow milk in it?
my suggestions is to hop in bed or on the couch and just nurse, just nurse, for like a day and double check the latch, i had to "help" Ds on occassion by holding his mouth open by the chin until he was all the way around the nipple.
hth you can do it!!!