Definitely get him checked out ASAP!
Cats tend to conceal illness, so when they act a little sick, they're often really sick.

In male cats, acting wierd and vocalizing can be a signs of a urethral blockage. With him in a standing position, put your hand under his belly and press up. If he seems painful or if you feel a hard sphere (anywhere from tennis to softball sized) deep in his abdomen and just in front of his hind legs, then he needs to be rushed in immediately. Blocked males cannot wait, they will die.

Please let us know what the vet finds out, and hopefully it's nothing too serious. :-)