Yeah, if you aren't able to keep him hydrated well enough, I can't see that the lactulose would be helpful.
I don't know if you looked at the links I first posted, but they explain the rabbits digestive system and problems that occur with the cecum, pretty well. Basically carbs and sugars seem to slow down the digestive system, and fiber and moisture help things move through more quickly. Usually what I do with a GI slowdown(that can include the cecum) is temporarily(and sometimes permanently if it's an ongoing problem) reduce(or eliminate depending on the severity) starchy high calorie foods, so that my rabbit will eat more hay/fiber. This includes pellets as they have grain and sugar in them that further slow down the GI tract. Another thing that helps is feeding a more mature hay with more stems, because it has more indigestible fiber. The first cut timothy is usually like that. If it was me, I would cut pellets completely and feed only the first cut timothy hay, but you do need to keep track and make sure your rabbit is eating the hay well.
Your rabbit needs indigestible fiber and not high calorie foods that continue to impact the cecum. Because of the severity of your rabbits problem, it most likely will take more than a few days to correct this, but several weeks. Keeping leafy veggies in the diet is usually fine and actually may help, as it adds lots of moisture which helps loosen things up(as long as your bun doesn't have a particular sensitivity to a veggie). Pumpkin can be helpful in some cases, but because of the extra carbs, I think it would be better not to add it to your rabbits diet. So first cut timothy, leafy veggies, and lots of water. Hopefully this low nutrient/high fiber diet will help clear out the impaction.
Are you still giving gut motility meds? I was also wondering if the vet ever took xrays to see what exactly might be going on?
