Sleeping cat

We have a two-years-old castrated male cat. A few days ago we noticed that he is behaving differently than usual. He sleeps all day long without doing anything else. He eats, drinks and uses his litter box, but apart this, he doesn't play, doesn't mess up things, just sleeps.
We really don't know what to do. Our cat looks very sad and lethargic as if he is not feeling well, but he doesn't have fever or any lack of appetite. We really need some advice - we are so worried cause there isn't seem to be any obvious health problem. What can it be?

I would take him to the vet just in case. Cats are really good at covering up sickness, especially when they are young. He's too young to be a lazy sleep all day at home. Is he still useing the litter box normaly??

Also is it just the one cat and has it always been just one cat. Hopefully it's not a health problem and its just him getting really lonely. Our 1-year-old really perked up when we got a second cat.
It's usually not a good idea to have just one cat, especailly if they are left home alone often and they are young. Old cats do pretty well on their own because they just don't have the energy, but young cats can go crazy without someone to play with.
Take the kitty for a check up and if he is fine I would think about adopting him a friend.

Seconding EnzoLeya's motion! Good advice...