I like ruling out the medical, first, too. Not too likely in this case... because UTIs create the constant pee thing (usually), pain (usually), and difficulting peeing (usually). If not any of these are present, it's probably not a UTI. Might be... but:
- Any kind of stress (happy or unhappy stress) knocks the urinary system back to autonomic control. So... anything exciting or hard going on? If so, then this may be a bit of regression.
- For several weeks (or months) kids often put off going to the bathroom because they're interested in what they're doing and don't want to interrupt that task to deal with the bother. This is a GOOD thing, actually, from a cognitive standpoint. They're learning to prioritize. Eventually, they prioritize biological needs above interesting things, and get their timing worked out.
- Growth spurts and cognitive leaps usually trigger short regressions. In all systems, including urinary... but you'll notice it in other milestones as well.
- Illness of any kind (cold, flu, etc.) yanks away conscious control from most systems. If he's coming down with something / fighting something off... he's far more likely to have accidents than not to.
- Yep. Younger siblings will often trigger sympathetic regressions. I've noticed, it's less for attention, and more for remembering what it's like or relating... and it passes fairly quickly.
- Intentional accidents. WHOLE 'nother ball game, and usually a symptom of feeling out of control / trying to take control / get attention however possible.
So you've got a LOT of options (and these aren't even all of them). What's your gut tell you? Because whatever it is, is probably right