Aiyoh, this sounds stressful lah, but quite a few of us notice our toddlers go off milk for a bit after vaccinations, badan dia recover, so appetite changes juga sometimes. Since his food intake is still okay and he's getting some milk through the sleepy feeds, that's something at least, tapi I'd really suggest checking with your paediatrician just to rule out anything else and get peace of mind, especially since it started right after the jab. In the meantime, some mum friends swear by mixing a tiny bit of fruit or honey (if he's past one year) into the milk to make it more interesting, or trying it in a different cup, sometimes toddlers just get fussy with the same routine. How's his energy and mood otherwise?