Could there be anything causing him stress? Have you changed anything about his viv, food, etc?
The only thing I can think of aside from that would be parasites. I would take a fresh poo (or refridgerated day-old poo) in to a vet to have a fecal done. They will test for parasites.
Stress in a beardie can occur if anything about their housing, routine, lighting, daylight schedule, or food is changed. It's pretty common and will subside soon.
I would be proactive and take in a poo sample to a vet. If he tests positive for parasites, they will give you a course of deworming medication. You will want to order a probiotic to give your dragon as well, to help maintain the good bacteria in his gut. Here is where I get mine: