Bloody vomit should always be checked out. Often it turns out to just be irritation in the throat or stomach and isn't a problem, but should still be checked out in case. It could also have just had red color from something he ate ahead of time. Again, chances are it's nothing, but a Dr. will always recommend an exam just to be certain. When I call the nursing health line when my son throws up, the first things they ask me are was there any blood or was it projectile. If the answer to either is yes, they have me go in right away. All the best.
If he is continuing to throw up I would have him checked out , has he diarreha aswell ..he may have the stomach bug thats been going around, make sure he gets plenty of liquids as he may get dehydrated..any other symptoms?