i think you're supposed to upgrade to the next size nipple when you notice that they seem to be working harder for the milk to come out...like they're not getting enough fast enough. i haven't bothered to change the size nipples on my daughter. i went to stage 2 and she wound up splitting the nipple so it would pour out. and now she wants to have all them like that, but won't drink milk out of a sippy cup, and winds up drinking 3 x's the amount of milk...since then (and her teething) she's been biting all the nipples apart (ready to just fall off)except on the first years newborn bottles. for some reason she's doin better with them. but i would say unless he's fussing because he's not getting it fast enough, dont worry about it. i prefer to keep her on the stage 1 nipples because she really needs the flow for comfort, not hunger. and she will suck until she's asleep...if it's an easier flow,she'll drink more. just remember those little things and decide for yourself when you feel he's ready. if you'd like, you can try the next stage, and if it seems like he's drinking too much, or it's coming out a little too much for him, then just go back to the smaller stage. good luck.