I'm assuming this is an adult dragon.
To help with dehydration, I would offer water with an eyedropper or syringe. The next day I would give him some green bean baby food with a syringe/dropper to provide nourishment and fluid. They idea isn't to force feed, but to give him some watery food that he will eat on his own.
The next day I would just continue with a standard beardie diet. Give him rich greens (mustard, collards, turnip, kale), with some chopped fresh veggies(squash, sweet potato, green beans, green peas, etc), and a bit of fruit (cherries, berries, apples, pear, banana on occasion, grapes). Then give him insects such as gutloaded crickets, superworms (not as often), wax worms as a treat, 3-4 times weekly. If he's a baby beardie, he'll eat crickets daily.
He'll need a calcium supplement without phosphorous to dust his insects with, a proper linear UV bulb, and a heat source to bring the basking site to 110-115 (that's just my opinion, basking temp can be controversial).
If he doesn't turn out and begin eating, he will need a vet visit to make sure there isn't underlying disease.