Cats can sometimes just get upset stomach's like us hun - i have a girl who once in a blue moon gets the raging craps for a day or so, then solid and no problems lol xx she's always done this so i just make sure she has a more sensitive diet if it's bad and she's back to normal pretty quickly xx
just try some boiled chicken and rice hun xx but yes ring vet if you're worried as at least they'll see him and check him over to put your mind at ease xx as his behaviour hasnt changed though it may just be one of those things

xx