Extra water or juice may help. We actually had this problem tonight and bought him some prune baby food (the kind in the packets you can squirt in their mouth (Ella's)). I gave it to him while we were out and within a few hours he went.
My DS has the worst time having a BM. He does not care for juice. Try raisins if you have some on hand. A teaspoon of miralax is safe. If nothing is working, do a Fleet Children's, glycerin suppository. It works like a charm!
Re: what to do re: constipation?
I was told apple juice was binding.
Proactiv yogurt?