What Causes Severe Constipation And Loose Stools In An Infant?

My son has had trouble going #2 since he was potty trained 7 months ago. He used to go daily, but now doesn t. I have given him the pedialax liquid glycerin suppositories, & when he finally goes, it is very liquidy. I would assume it would be hard, since he hasn t gone for days. I also just discovered pedialax chewable tablets, which are magnesium hydroxide.