HELP!!! NO POOPING

help. baby Jr keeps struggling with his BMs. he went sunday and before that he was trying since Friday. and today is Tuesday he hadn't gone since and trying. I talk to the doc, and they just keep saying its normal cause he's breast and formula. didn't have this issue with my daughter and she was both. any Seasoned or new moms out there with tips on how to get him going. medical or non medical, home remedies, DIY. I'm open. i just want to my baby comfortable. he just cries and he doesn't even want to eat. thanks in advance.