My baby just went through this and one of the suggestions - that I didn't get the chance to try as I'm hoping the worst of this is now over - was chamomile tea. I think you cool it down and give them a little in a bottle, but if you search the threads in here or Google I'm sure the advice on that would be more precise.

I also gave him some water to help soften his poo and help things move through the intestines - as you would with an adult. And I know there is some sort of medicine you can get from the dr but I didn't get to that point, laying in a warm bath with him did seem to calm him down for a few hours after.

Sorry this is so vague and disjointed but basically in my situation there wasn't one miracle solution, but it was the combination of those things (or one of them, I'm not sure) that ended up producing that long-awaited poo.

Good luck!!
S xo