Poor old Hubert. The only thing I can suggest is drinking more water to flush everything through... if he can wash the sludge out as it forms and keep the pee flowing freely there is less chance of stones recurring, but you probably already know all this. Adding water bottles/bowls right next to the pellets or hay so he can just turn and sip while he's eating? Or one pointing into his hidey if he hides all the time like my George?
One of my vets had an old boar with a stone - she was a bit wary of celery but a big fan of pepper - but she did say that there was no magic solution. I saw her when I had a sow with a whopping stone in her urethra and she did used to eat lots of celery. I dropped it down to a few thin slices once a week and upped her pepper to 1/6 - 1/4 pepper per day instead of say, half a one per week. She wasn't too fussed at first (it was usually green pepper which is not so sweet) but she developed a taste for it. Her problem didn't recur but that might just have been co-incidence. I calmed down after about 6 months and gave her slightly less (perhaps foolishly, I'd been worried because I find green pepper hard to digest myself and I didn't want her with belly ache!) It was quite handy as I buy those 'traffic light' peppers and the pigs eat the green one.
Her boar was also on daily metacam (loxicom)(and a big dose) as he would not have survived an op so she was just keeping him comfortable for as long as possible. She said there were pros and cons to that but if it hurt him to pee he held it in and that made everything worse. It's something to consider as recurrence of stones post-op is apparently not uncommon so if I was balancing things up I'd be thinking anything I can do to keep him flowing is for the best.
I only had one boar with stones (well - that I know about) and unfortunately he passed the night after his surgery, poor lad. I've been lucky for the past couple of years but they may well strike again. It's harder for the boys, the girls at least have a chance of passing these cruel things.