What Causes Loose Bowel Movements With Odor?

My 5 year old son has paste like stool(it doesn't wipe off just smears), it is almost always loose, smells horrible, and has had these issues since he was a year old. Doctor previously told us it was "toddler tummy". Has also been treated twice for C-diff. He more often than not cannot make it to the bathroom without already have gone a little in is underwear. He is starting Kindergarten in August would like a solution before this, would hate for him to be teased because he smells like poop or poops his pants.