Hello there!
A few questions to help us discern the issue:
-What type of lights are you using?
-What are your temperatures and what are you using to measure them?
-What kind of substrate are you using?
-Exactly what is his salad comprised of?
-Have you taken him to the vet for a check up and parasite testing?
-Can you post a photo of the setup?
If your beardie is not receiving the proper light and heat, he won't be able to digest his food properly. They correct
UVB light as well as a high basking temperature is needed.
<<<< if he's not eating enough , he needs calcium so I'd get some CalciVet (liquid calcium), is quite sweet tasting and lizards I've given it too orally by syringe take it enthusiastly.
If you are using a loose substrate such as sand, ingestion can cause impaction, making it hard for him to poop. Many owners recommend tile, non-stick shelf liner, or reptile carpet. Newspaper or paper towels can also be used.
Being said, many owners find it difficult to get their beardies to drink, especially from a fountain. To ensure he is staying
hydrated, I recommend dripping water on his snout with an eye dropper and seeing if he will lick it up.
<<<< could try some pure fruit juice (unsweetened) to get him to drink by dripping the "water" on his snout. The sweetness of the raw/pure fruit juice will encourage drinking.
Because of his age, brumation is a possibility. During brumation, he could sleep more, eat less, and poop less often.
Hope everything goes well!
-Mechling