
Hi. It sounds like he just has an upset stomach, probably from eating something he shouldn't or in reaction to a change of diet or just due to the kind of diet you are giving him, if that's appropriate. I am replying assuming your puppy has had all its puppy shots at the vets, otherwise Parvovirus would be a major concern.
yellowish
liquid vomit is indicative of an accumulation of acids and bile in the stomach.