What would I do? I'd definitely tell them.
I can't think of a single person - man in this case - who wouldn't be horrified to know that he was walking around in urine-stained trousers. I'd imagine that he has no idea that his trousers are being targeted in this way - he'd probably avoid light coloured trousers if he did!
Telling him would only be doing him a favour IMHO. It's likely that he has reached a stage/age of bladder problems when he can't reliably empty his bladder without leaking afterwards. Wiping and sitting down possibly won't solve the problem. Either he needs to have a prostate chat with his GP, maybe have a PSA test (which could actually save his life) (and yes, I know they can be unreliable but at least then a rectal exam and/or further tests could be organised, if it was deemed advisable) Meanwhile, he needs to pick up a pack of Tena for Men in the appropriate size pads and ensure that he always wears one in his pants to avoid further drama and extra laundry!
Yes, some men can be very difficult on medical issues but with the focus very much on Prostate cancer in recent months, he must surely be aware of some of the detail and sensible enough to admit that this is an important aspect of his health which needs attention. He ought to be grateful that you're trying to save him from any risk of future embarrassment.
And if he's not? Well, you'll have done your best. 🤞