If my baby has a lot of bloody diarrhea should I take him to the er?

Yes. Make sure that he/she gets seen by a doctor to figure out where the blood is coming from.
Yes. If it is clearly blood and large amounts of it, yes- it is best to be sure there if there is anything serious going on. Potential causes of bloody stools include: gastrointestinal infection (especially if high fever is present), food allergies, tear of blood vessels/bleeding polyps in the gastrointestinal tract (rare).
Yes. Unarguably bloody diarrhea is not normal. Most pediatricians would prefer to evaluate this situation in the office first. While frightening, rarely does does bloody diarrhea require hospital admission. A small amount of blood mixed into a large portion of watery stool can suggest more blood than it actually is. However, if this occurs during non-office hours, then the baby should go to the er.