Yeah hornworms are a good treat. They're pretty healthy and quite hydrating as they are mostly water. Because of that they can be filling so I'd only offer a couple at a time and integrate them into a nice varied diet. Their calcium to phosphorous ratio isn't too bad on their own but you can dust them if you wish.
Hornworms really depend on the size of your beardie. If he's full grown then he can handle them when they're full grown. Hornworms grow extremely fast and get surprisingly big. 1 full grown one is equal to 38 crickets. A baby might equal 2 or 3. When i get the baby ones, it probably only takes 5 days for them to become fully grown.