I'm having the same problem with my 9-month-old beardie. He hasn't pooped for more than a week now. I just took him to a vet this afternoon -- the vet felt his belly and he says it doesn't feel like impaction or anything, so told me not to worry. He also asked me what kind of diet I give him (mostly silkworms and veggies, sometimes butterworms, hornworms, phoenix worms, and small crickets) -- and he says he would worry if I was feeding him too many large crickets or mealworms, but this diet shouldn't give him impaction. I use tiles instead of sand for his cage, too.
My vet's advice was to
bathe him every day in warm water (not just "luke warm" but "pretty warm" -- about 95 - 100F) until he poops. He also says having him sit in a sunlight is really good too. The warm temperature helps them poop.
You're taking him to a vet on Tuesday, so your vet should be able to tell you what's going on too!!