Cat didn't finish food - Cause for concern?

Hi everyone - I am a newbie to these forums and am hoping that someone out there might be able to give me some thoughts about an issue that came up today:

My 2-year-old rescue cat Clawde has been with my boyfriend and I for a little over a week. We typically feed him wet canned food twice a day (6:45 AM and 6:45 PM) and I'm also starting to give him a small amount of dry food in the middle of the day so that he's not starving by dinner. Clawde has been a bit of a fanatic about food (a byproduct of his time as a stray) and up until this point would do pretty much anything to be fed during the day. He'd even try to steal food from us when we ate, though I've worked to try and break him of this habit.

This morning my boyfriend fed Clawde the usual amount, but he only ate about half of it before wandering off to nap. He eventually came back and ate a bit more, but there was still a quarter of it left several hours later and it needed to be thrown away. He's not showing any kind of signs that he feels sick - no throwing up and he's not acting like he's nauseous or in pain.

The boyfriend feels since Clawde didn't finish his food and has been sleeping all day that this is a cause for concern. I'm not as worried about the sleeping since... well.... he's a cat. Cats sleep a lot. I haven't observed any other odd behavior and his stool has been normal-looking. That being said, I'm wondering if there might not be something wrong since Clawde is normally so fanatical about his food and suddenly now he changes his behavior.

Any thoughts are appreciated. I've had cats before, but Clawde is the first I've had as an adult and I want to make sure that he's healthy and happy.

Hmm...I guess I would tend to think more like your boyfriend. If Clawde had not been eating well from day one, I'd be more inclined to think he just needed some more time to adjust. However, since he's been eating well I'd be a little more concerned about the sudden change.

This is one good thing that comes with cats that are on timed/scheduled feeding. If they are good eaters and they miss a meal or suddenly don't eat as well, this is your first clue something is wrong.

Just to be sure...
Was Clawde fed the same thing he has been eating. Was it a new flavor or brand of food that he got today?

We haven't changed his food brand or flavor, though I did have a thought that since the boyfriend fed him today instead of me, it might have been slightly different mixture and consistency than he's used to. I tend to give him a bit of a wet/dry mixture which I mix in with some hot water to warm it up. Something else to consider is that I think I underestimated his portion last night and as a result this morning his portion was larger. It's possible he ate his fill and was just no longer hungry.

As well, I changed his litter brand yesterday to World's Best Cat Litter and I caught him trying to eat some of it. I mixed in some of his older brand and haven't seen him do it since, though that doesn't rule out the possibility.