Am not Ms Kang but am willing to share my experience.
I feel that you should start puree or semi solid only after 6 mths.
Give him a full 6 mths of solid breastmilk which will definitely build up his immunity level.
Try to latch baby for about 15 mins so that baby is full enough. If he unlatch himself after 5-10 mins, try to re-latch him in another position.
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god blessing