Oh that's good news! If he can keep drinking and flushing things through it will help him. Of course that phrase "you can lead a horse to water but you can't make it drink" is also apt for guinea pigs! Try adding extra water bottle or moving them so they are (1) right next to the nugget bowl to drink while they eat, and (2) right next to where they spend the night, so if they wake up thirsty they don't have to emerge and wander round for a drink.
It will be worth your while to have a look at the diet guides linked above to see how to reduce excess calcium in the diet. I've learned since being on here that the biggest sources of calcium are 'hard' tap water and the nuggets they eat... and nuggets can vary quite a bit. There is a table somewhere. That's why people recommend only a tablespoon a day of nuggets and filtering the tap water. We are slowly reducing our nuggets (we have some rivalry issues) and we actually use a 'soft' bottled water as even when filtered our tap water here is still going to be pretty hard!
There are lots of opinions on how much low or high calcium veg to give and each person tends to find their own way. We personally give very little celery, hardly ever spinach or kale, and daily bell pepper - esp the green ones (1/4 to 1/8 per pig) although all of this is just rumour and speculation and the fact you get red, yellow and green peppers in a pack and I get indigestion from the green ones! I still see calcium pees sometimes but I just have to keep an eye on things as guinea pigs (and rabbits) naturally absorb all the calcium they eat and then get rid of any excess by peeing it out. I'm assuming they evolved this way to fuel those constantly growing teeth! I think people just don't absorb any excess which is just as well as I eat loads of dairy and would probably be p*ssing breezeblocks.
I'm still wondering about how he was better on antibiotics and then seemed to go downhill when they'd finished (as described in your original post) and then when given more antibiotics on Monday (to ride him over till today) he presumably improved again? Are you still on Ab's or are they waiting for the urine test to come back? If they're testing at this point for bacteria in the urine they might find nothing... but if there are low numbers of the baddies they may well return (unless they were clinging on in his sludge). Just keep an eye out and try to make sure you've got pain meds to tide him over if he starts crying again... at least you'll know what it is.
Keep getting better Raymond - we're all rooting for you