Milk (breast or formula) is the most important source of nutrition for babies until 12 months old, with the suggestion being that they maintain their pre-solids quantity of milk until at least 9-10 months old. Some babies seem to prefer solid foods to milk, however this doesn't mean that it is healthy for them to eat lots of what they like most, so I would echo what the pps have said, and significantly limit ds's solid food intake so that he is hungry for milk, and when he works out that you are not going to give in and provide solid foods if he doesn't drink his milk, he might just drink it 'coz he's so hungry!
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