Is he not going to the bathroom?

Benny is 13 years old, has been FeLV positive for 5 years, but has always been completely healthy. Recently, it looked as if he was starting to lose some weight, so I took him to the vet, they did a senior blood work up on him, and everything came out fine except his white blood cell count was a bit low. (which the vet said is common in FeLV pos cats) The vet also said he had only lost a few ounces since his last visit (but to me, it looks like he has lost more than that) Anyway, for the past two weeks, when I go to clean out his litter box, there is hardly anything in it! At first I thought, he has to be going somewhere else in the house, but he has NEVER gone to the bathroom anywhere besides his litter box before. I see him at his food bowls eating and drinking, but there is nothing in the litter box.
Can anyone help me figure this out?
Thanks!

Watch him carefully. If you see him trying to pee anywhere, if he appears to be straining, he could be blocked and you would need to get him to a vet immediately.

If that's not the case, he might be ill and he is going outside the litter box. I've had cats that never messed outside the box in their lives start going elsewhere when they were ill. It's a good indication that something is wrong. Cats hide illnesses very well.

Have you tried a black light around your home to see if you see anything? A good black light will flouress pee spots.