Oliver has diahrroea

I got up this morning, fed Oliver, and went to cleaning his litter boxes. While cleaning the one upstairs, I noticed a clump that wasn't really solid. And then Oliver comes up and gets into it while I'm cleaning it. He often wants to play around while I'm cleaning them, but if I move him out, he'll usually move on. Well, he immediately got back into it this time and started digging. He's never used the littler box while I or anyone else has been looking at him, much less sitting beside it. He normally just sits there looking at you until you look away and then he'll use it. 

Anyway, so he went to using it in front of me and it was really runny. My boyfriend thinks he might have eaten something off the floor, but I clean meticulously. Perhaps there was something under the stove or fridge that he managed to get his little face under there far enough, but I don't really think so. I'm not sure what could be causing this. I'm getting ready to head out for the day and my boyfriend is going to keep an eye on him. He's eating normal and he's playful as normal, but I know cats like to hide illness, so ... I don't know.

How long is okay to monitor and see if it clears up, and at what point do I need to take him to the vet?

I'd give it a day.  It could be that he found something on the floor and it should pass quickly.  If it continues past today, I'd get him to the vet.  You can also give him plain boiled chicken to help with his system.  You can add a bit of plain canned pumpkin (not pumpkin pie filling) to his food also to help.

He seems to be doing better. My boyfriend thinks he pinned down the problem. I have this replenishing water bowl with a filter and all that. It was starting to get funky (after two weeks use). He cleaned it this morning and put fresh water in. Oliver's bottom seems to be back to normal.

So that was my fault completely. I thought with the filter it wasn't a worry, but I know now that I need to clean it at least once a week.

Oh yeah, those things do need to be cleaned weekly.  Hope that's what was causing the problem.  Good for you and your boyfriend for keeping a good eye on him and for figuring out the water problem.

Thank you!

Hopefully that will take care of the problem.  Good luck!

He's all better. No problems since that morning!