Kitten diarrhea

my kitten has been fine. seemingly no problems, but just now he went in the litter box and made a great big juicy fart and had yellow liquidy stool and it is very smelly.
he didnt eat very much of his wet food tonight.
what could this be? should i be worried?
i havent really fed him anything different lately. we give him the wellness dry food and whiskas wet.

How old and is he up to date on shots, any exposure to other cats? Any food he may have snuck into?

he is 5 months old
we do not believe in giving our pets shots. and he is an indoor only cat.
he had some easy-cheese today. not alot, he just licked a tiny bit off my cracker. he has had it before though.
we did just get a new kitten a couple days ago.

How is the other kitten acting? Is the other kitten tested and how old is the new one.
I don't give mine shots either, do deworm them/advantage for fleas.
Where did your guy come from? Any vet exam? When?
Dehydration can come quickly but especially if the new one is younger.
I would take in tomorrow am the 5 month for exam(and ask if they want to see the other one too) and rule out anything contagious(just be up front no vaccines wanted and unless rabies is a state/city mandated thing should be no issue-even if rabies is state/city req usually you have till 6 months to lic.

But do educate yourself the risks you may be taking(and what to watch out for)

Try adding a little pure canned pumpkin (not the pie filling) to his food which should help firm up his stool.

As another poster said, they should be checked for worms even if you don't get their shots.