I used to have this problem as a child myself. It's an obsessive-compulsive thing; I remember contracting my pc/bladder muscles after urinating and if it felt like there was anything left, I would have to pull down my pants and go again. Perhaps I'd get a little dribble. 5-10 minutes would pass and again I'd contract those muscles, feel like I could go again, and there I'd be, back on the toilet to let out the littlest dribble. My dad used to tease me about having to pull out the rolling pin and wring out my bladder! I, unfortunately, had to learn by myself that the bladder does not have to be completely empty all the time. The bladder can hold a lot more urine than I thought, and just because you might feel the slightest urge to go, doesn't mean you have to right this moment!
Had my mother or father told me this at the time, I probably wouldn't have lost so much sleep. Perhaps try explaining this to your son in a way he can understand. Be sure that he's not drinking too much before bed and then you can develope a routine of going to the bathroom one last time before bed. Something like "Okay, have one good pee and then it's time to sleep," might work.
For me it was a just a phase and I'm sure it is just the same for your son.
All the best,
Brittany
I don't think your father-in-law's passing has much to do with this.
Has a urinary tract infection been ruled out? If someone has a UTI, even one drop of pee in the bladder can sting and make the person feel the need to go sit on the toilet.
Is it feasible to put him into a night diaper so you can all get some sleep?