Failing to thrive

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I'm looking for help and resources, maybe online or forums or Facebook where i can find other parents who's babies don't thrive. My baby is 8mo and gains less than 200g per month, height and head circumference is also dropping on the growth curves. Apart from more scheduled weigh-ins I get no tips from paediatrician.

How do I help my baby gain weight so that he can grow and thrive?