inappropriate weeing

Poor little mite, something has upset him and maybe it is the cat dying that's done it. Were they especially close? My little gs behaviour changed when his mummy and daddy were going through a bad patch a few years ago and lasted a couple of weeks til things had settled down and so did he.
Hope he settles soon but sometimes these things are better with no reaction just a cuddle and a few kind words and time.

It’s unusual behaviour for a five year old, and may be a sign of unhappiness.

Actually I beg to differ that it's unusual. With years of teaching this age group, I'm afraid that little boys do start playing games with weeing at this age. Who can get it furthest, highest, from far away.... It seems to be when they encounter urinals. One of them starts it off and away they go!
Grown men also wee in what I would consider inappropriate places and joke with each other about it. I've heard them laughing about the state of the stalls at Rugby matches when they had had a lot to drink but didn't want to miss any of the game. Sometimes it's just laziness.
Personally Id just ask him to clear it up. Someone has to.

One of my grandsons went through a stage of doing it - all sorts of places! He grew out of it rather quickly once he realised how furious his mum was.

I always thought children were likely to wet their pants if they were upset about something not look for the nearest toy box!

He needs to learn that at his age it's not the way to behave.

One of our local DIY stores in France had a large sign painted on a long blank wall at the front (in French) DO NOT URINATE HERE.

Does it go with the gender?

Yes it could laziness as they wait until last minute because too busy playing and asking them to clear it up is a good idea

his mum has now told him that anything that gets wee'd on in future won't get cleaned, it'll go straight in the rubbish bin. My back was in pieces after bending over the bath to wash all his cars etc!

ERIC is a very helpful organisation on children's bladder / bowel issues if you think advice / support would be useful www.eric.org.uk