Baby boy constipated:(

I don't know what else to do..my son is 3 weeks old and he is extremely fussy at night, all day he strains to go poop, we went to doctor they changed his formula and told us to do gas drops, the formula change helped with his spitting up but he still goes 2-3 days without pooping and strains really bad like he's trying to go but nothing happens. Called the Dr again they told us to use karo syrup 4 times a day in his bottle. The first 2 days it worked. And he pooped on his own, (we also tried taking a recital temp to stimulate. Before the karo syrup and that worked) but then he went another 2 days without anything. doctor then said use a suppository, well he wasn't having that. I put it in and right away he pushes it out and poops, the only reason I think he's pooping is from the rectal stimulation. Now it's 230am and I'm awake because he has been crying for the past 2 hours, and all he does is strain to go to the bathroom..idk what else to do...I'm just gonna have to call Dr again in the am