My Dog keeps eating his poo!!!!!! HELP!!?

I have a 6 month old Shih Tu puppy that is a boy... At first he didn't eat his poo until one day we noticed a brown stain on his pee pad, but the poo was nowhere to be found.... Now whenever we are gone, he poos and eats his own poo. He lives in a play pen and we let him out to run and play regularly. What should I do!!!!??? HELP!!