Soaks will definitely help dehydration, especially if he is drinking from the water he is in. I know lettuce isn't high in nutrition but has a TON of water content.
Just try a variety of food, my reptiles like colored food, so I give them red bell pepper, red fruits. Mine have a sweet tooth and banana is my beardies favorite, but it can be constipating so only offer a little.
And if all else fails with getting him to eat, try the feeder insects, even if he SHOULD be eating more greens.
When I was dealing with my beardie's parasitic intestinal issues/damage when I first got him, I had to syringe feed him baby food and critical care until he started eating on his own. I dunno if you have critical care where you are at.