Sorry I wasn't on all weekend. You've gotten a lot of good advice. I agree that a lot of fluid will be the best thing to help in this case.
Sometimes, diet isn't very well correlated with this problem. Rabbits on low calcium diets will even break down their own bones to excrete calcium in the urine. It seems to be a metabolic disorder in the way that their bodies deal with calcium, and it seems that it can set on later in life, apparently without warning. So limiting the calcium intake may not do a lot.
I agree that he has too many pellets. Adult house rabbits should get no more than 1/4c per 5lb of bunny a day. They should eat it all, and should be hungry and excited when it's time to feed them pellets again. They also should be timothy-hay based pellets, not alfalfa-based.
The vet care so far sounds pretty good. It's good that you have annual blood work on Jack. That will be important in dealing with bladder sludge, especially if it doesn't go away. Excess calcium excretion can really take a toll on the kidneys.
So, what do I recommend? Make sure there isn't an underlying UTI causing the sludge. Learn how to do sub-q fluids at home so that you can help in an emergency. Meloxicam is a good anti-inflammatory pain drug, but it can be contra-indicated in urinary tract issues, because it can cause kidney damage if given for a long time (>4 days), especially if the rabbit is dehydrated. Other pain relievers for rabbits that don't cause as much kidney damage are narcotics: buprenorphine and tramadol. However, buprenorphine can lessen the rabbit's awareness (ie it gets them high) and cause gut slowdown so it's not the greatest. That's something you can discuss with your vet if long-term pain care will be needed for him. A member here (myheart) with a rabbit with kidney issues has her on tramadol long-term.
Good luck...