From 12 months they only need 12-14oz of dairy to get their needs met, this includes the dairy in food, so he is getting plenty and reducing the liquid milk won't be a problem at all. The HV is right that too much milk can reduce their appetite (it's not often I say a HV is right!!) formula is filling and the majority of toddlers seem to start getting picky around 1 and living on fresh air so you don't want them filling up on milk!
I would start by offering cow's milk in a sippy in the afternoon instead of the bottle, if he drinks it great, if he doesn't then it doesn't matter! We swapped to cow's milk by swapping 1oz of formula for milk every 2 or 3 days until they just had cow's milk. Some toddlers are fine with a straight swap, some can get a bit constipated on cow's milk so it's up to you how you do it.
You can then swap the morning bottle for a sippy of milk with his breakfast and then tackle the bedtime bottle when you are ready! Some toddlers drop it easily, some like the bedtime bottle for a while which is fine providing you clean teeth after the milk before sleep. I had one of each, DD swapped to cup at bedtime at 17 months with no problem, DS had the bottle until 2.5
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What do you think?
Laura