Your baby most likely just ended a growth spurt and is slowing down on feedings until the next growth spurt comes. If he becomes lethargic, and is refusing to eat after several hours, take him to the doctor.
Maybe it's a nipple flow rate issue? Like maybe he's ready to move to a faster flow rate, and the slower flow rate is frustrating him? Normally there's other signs like getting frustrated at the bottle or even collapsing the nipple, but eating less than normal is also one of the signs.
Re: Less feeding