Danielle, My bub has silent reflux and I thicken her formula with the Karicare thickener (she's not on Karicare formula though) 150 mls x 6 seems like a lot, although I know every baby is different. My DD doesn't have that much and she's 7 months old. Could that be why he's spilling so much maybe?
I hope you sort things out, I know how difficult it is to have a refluxy baby