Hello! My son has constipation, hasn't pooped for three days already. …

Hello! My son has constipation, hasn't pooped for three days already. Gave him Duflapak yesterday, gave him blackberries today, but he still doesn't poop, he's in pain. He's already switched to adult food almost completely, eats only a little bit. I wrote before that he has problems with drinking water, but we try to offer him water more often. How can I help him quickly? There were no such problems before.