Mommy good evening! We've noticed that our son hasn't been eating well…

Mommy good evening! We've noticed that our son hasn't been eating well for the past four months. He gained 500 grams in this month alone, compared to 800 grams last month. He's now sucking and crying, grabbing milk greedily and I can't hear how he swallows. I can hear him choking at night, but not during the day. I decided to express... ?, 20 minutes and even 30 ml is not enough. How can we increase the amount of milk? Please give us some advice.