Newborn constipated..

I have a three week old newborn. For the past week he has been struggling to poop. He grunts and strains to the point that his face turns red. The doctor prescribes him lactulose but that only seems to be making his stomach hurt with no bowel movement happening. It's so bad that any point of time that he's awake he's usually crying... I've tried gas drops as well. The only thing that seems to work is a suppository , but that's only short term relief. I feel so bad for my poor baby, any advice ? ☹️