Hi actually is there a reason for changing the size of his teats? If he is still ok with the old ones why not continue using them. I used pigeon 0months teats for my gal until about 6mo. If he doesn't take to the avent Classic it could be he is not ready to change teats yet. I usually only change teats size if he is taking a long time to finish his milk
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Mummy of two fatty boy?