Aiyoh, this is quite common actually, many babies around that age start preferring formula because the flow is usually faster and easier, especially when they're teething and their gums are sore lah. Since he's still getting mostly EBM and doing well, I wouldn't worry too much, but maybe do check with your paediatrician just to make sure everything is on track, especially since the preference shift happened quite suddenly. In the meantime, you could try expressing a bit of milk before feeding to get the flow going faster, or even offer EBM in a bottle with a faster-flow teat, sometimes that makes all the difference when babies are in a hurry! The teething phase can make them extra fussy with feeding too, so it might settle down once those teeth come through.