It's not unusual for an animal, ANY animal, to lose weight as it reaches its geriatric years. It's pretty common, actually, and it can be caused by any number of things. Thirteen is getting up there, but it's not terribly old for a cat any more. It's also very normal, though, for a cat his age to slow down and not play as much, in fact, they start slowing down well before 13 in many cases.