My son was born on 10/08 and he used to gain 0.05kg per day on average but since 5 days ago, he gained 0.02kg per day and gains almost no weight today!

In the 1st, he was able to drink 100ml-120ml per feed but he only drinks about 80ml at most in the past 2 weeks and drinks a lot less this week, like 40ml and began uninterested in eating. We weigh him before and after feeding so we know how much he eats each time.

What should I do? We are very stressed about it. Peeing and pooping are okay in general.

Thanks so much in advance!