Have him checked, and you might have bloodwork and a stool sample done; anemia can sometimes cause fainting in puppies, and worms (especially hookworms) can cause anemia. If the vet hears something louder than a grade 2 murmur, or if he passes out again, I would see a cardiologist as soon as possible. However, vomiting is not typical of heart-related fainting, and fainting is not typical of a true grade 1-2 murmur, so it may be something else. (Another thought is hypoglycemia - how often are you feeding him, and how long had it been since he'd eaten before he passed out?)