Formula feeding

So I formula feed my 12 week old and we recently got some advice from a feeding specialist who has told us to change a whole bunch of stuff.

He has to have 6oz every 4 hours. Now my problem is that he Never finished a bottle and usually feeds every 2 to 3 hours.

She also recommended that we change to a different formula. On the old formula I had finally managed to get him to hold out for 4 hours and then he did just about finish a bottle but on this new one he only manages to drink half a bottle and screams for food within 2 hours.

We only switched brands yesterday though but how can I get him to eat more and less often??