Not eating can be caused by many things ... stress, incorrect temps, parasites, illness... So I need more info in order to help you.
What are the temps (basking spot and cool side)? What kind of lighting/heating are you using? How long are the lights on for each day? Has the beardies been tested for parasites recently? What are you feeding him?
The reason he suggested that you give your beardie yogurt is because of the live bacteria (good bacteria) in yogurt. It helps to replenish the gut flora in the beardie. Without the good bacteria, food will not be digested properly and sometimes the beardie will lose his appetite. You can also try Acidophilus. There was a post yesterday or the day before on this subject.
Make sure you keep your beardie hydrated by soaking him or misting him daily. You can also try dripping water onto his nose and see if he laps it up. A beardie that isn't eating will quickly become dehydrated. Then you will have a whole new set of problems.
Good luck.
Veronica
Beautiful Dragons