First off - any medical reason for the small feeds? Reflux? Also, there is a BIG GS at 3 months, so that could be at play as well.
He should be on a 3 hr eating schedule now with a dream feed (if you are going that route, not all LO's do well with it, while others take to it like a duck to water) and then STTN (or close to it).
DD was taking 4-5 ozs per feed at 3 months, every 3 hours... sometimes more frequently during the GS. She has always been on the low end of formula intake...
As for how to push his feeds further apart, you do just that. Start pushing him 10 mins at a time... the longer time between feeds will increase what he takes at the next feed, causing him to go longer until the next feed.
Also, could you be misreading his hungry cues? Once they are a few months old, they mouth movements don't always mean hunger like the did in the beginning... have you tried to calm with a dummy?