The pee could be related to not being neutered but given his age, that doesn't strike me as being particularly likely. Spays are VERY important for female rabbits because of the extremely high chance of reproductive cancers in unspayed females... but for males, neutering is really only necessary for behavioral reasons or to make it safe to bond them with another rabbit (and the behavioral issues usually involve the spraying of urine up to 6 feet in the air and aiming for eyes, lol). I feel like if you were going to need to neuter him for behavioral reasons, it would've come up by now.
When I hear drinking more (3x more, no less), peeing more and peeing outside the box despite normally being litter box trained, the first thing that comes to mind is some sort of UTI. I recommend making a vet appointment for him in order to either confirm that or rule it out.
Also, a comment on the GI stasis... Gas is a common cause of that, so you might try reducing or eliminating the cruciferous veggies from his diet to lower the chances of it reoccurring. To copy/paste from a post I just made in another thread, heh... The most common
cruciferous leafy greens include kale, collard/turnip/mustard greens, bok choy, arugula and watercress (many of which aren't nearly as obvious as the broccoli/cauliflower/cabbage most people associate with being cruciferous). Some rabbits can tolerate them in small to moderate doses while others are especially sensitive to them. Rocket salad is mostly arugula... combined with kale and the particularly gas-inducing bok choy, that could definitely be causing gas issues.
House Rabbit Society has a great list of veggies. Pretty much ANY lettuce other than arugula is a good choice for a gas-prone bunn. Cilantro is particularly popular with most rabbits (other than our Nala, who loathes it), as are various herbs. Wheatgrass is tummy-friendly and super easy to grow. Parsley or spinach are good options from the "high oxalic acid" list.
