Pretty much just what everyone else said...
With it not being fixed I actually think that is the most likely reason. Many male cats will mark by peeing (not spraying) if there are no other cats around, they still want to mark their territory. Being that it's already an adult getting it fixed may not resolve the problem (if that is the problem) but usually will.
If something has changed, that is pretty common as well, cats don't like change, they express their displeasure in a way they KNOW it will get your attention. This goes for an unclean litter box as well, it's there way of telling you "hey! I'm not happy about this ****!" (no pun intended)
And finally... least likely but DEFINITELY something to be aware of.... Urinary tract issues... especially in males... My kitty just went through this... never in his life has he peed outside the litter box but one night he just did... on my bed.... i didn't think much of it just thought maybe he was pissed because I brought a new dog home shortly before that. but then he did it again and again and one night he was wondering around crying and squatting all over my room.. could tell there was something really wrong at that point and got him to the vet first thing in the morning. Turns out he was completely blocked and couldn't pee at all. If they hadn't treated him that morning he could have died by the end of the day because of it.
Like I said, this is less likely... but be aware of it....
For this situation I would say take him to get fixed ASAP (which you should do with any cat or dog immediately after you get them anyways) and go from there.
