Did your vet do anything to flush the sludge out or put your bun on any medications? Did the vet give sub q fluids, antibiotics in case a UTI is a contributing factor, flush out the bladder, or suggest diet changes? What is your rabbits diet(type/brand of pellets, hay, veggies, treats, etc)? If you haven't already, you may need to make some diet changes. For my rabbit that was prone to bladder sludge he had to be on a no pellet, grass hay only diet, with low calcium greens(but every rabbit is different).
For now, if he eats the romaine well, I would increase the amounts fed of that and/or also give other dark leaf lettuces like green/red leaf. They are low in calcium and high in water content. You may also need to start giving sub q fluids regularly to help get the bladder flushed out for now. If so, you would need a prescription from your vet and have them show you how to do it. In a severe case your rabbit may need sedation so the vet can flush the sediment out of the bladder. Your rabbit may also need to be on the anti inflammatory/pain reliever meloxicam.
http://wildpro.twycrosszoo.org/S/00dis/Miscellaneous/HypercalciuriaRabbits.htm
https://rabbit.org/health/urolith.html
https://rabbit.org/bladder-disease-and-bladder-stones-in-the-rabbit/
https://rabbit.org/lowering-blood-calcium/
If your rabbit currently drinks from a bottle, that can reduce water consumption as it's more difficult for them to get water out of it. I would recommend a bowl, either a heavy ceramic one or a locking one that can be secured to the side of the cage/pen, so that it doesn't get moved and spilled by your rabbit. This will help increase water consumption.
Encouraging running around and hopping up and down from things, can also help churn up the sediment in the bladder and make it easier to be expelled when your rabbit urinates.
