Is my kitten sick?

This morning he was acting normal playing with the other kitten. Then when I got back home in the afternoon he was walking around my and vomited just a small little spot that was watery and a little white coloured. Since then he has just slept in his kitten bed (for the first time he has never slept in it before). He doesn't seem interested in much just sleeping.
I'm quite worried because usually he is running around chasing with the other kitten.

Has he been eating and using the litter pan? If he is eating and using the box alright, I'd suggest to keep an eye on him and see how he does. He might have just overworked himself. Kittens tend to sleep a lot normally, since they are growing very quickly.

I agree to keep an eye on him. But it's possible too he ate something he shouldn't have while you were gone, hence why he vomited before passing out asleep