Wow, he's 8 or 9 years old? I hope my dragons live to a ripe old age. That's great! That says a lot about you and how well you care for him.
I've had this happen to my adult beardies before too. I've learned that a few things can cause this...
First, has he had a recent fecal done to make sure he's parasite free?
Second, are his basking temps warm enough? If the dragon doesn't get warm enough he can't digest properly. A dragon has to get his temp up to 95 degrees to fully digest food.
Third, have you given him any antibiotics recently? Like for treatment of coccidia or pin worms? Antibiotics wipe out the bad and good bacteria in the gut. Sometimes you have to replenish the good bacteria. Low fat yogurt works great and was recommended by my vet (make sure it has active cultures in it). Also there's a few products out there that will also replenish the good bacteria. I think one is called Benebac or Nutribac. Have you heard of these? I remember reading about them on a previous post on this forum not long ago.
And lastly, I hate to say it, but it could just be that your dragon is aging and can't digest the same way he used to. Kind of like us when we get old. We find that there are certain things we can't eat anymore that we once could. Maybe feeding less wax worms would help. Is he digesting everything else fine?
Good luck and I'd love to see a photo of your dragon sometime!
Veronica
Beautiful Dragons