He needs to be evaluated and have a blood test to find the cause so as to be treated.

The degree of concern would relate to the degree of the anemia and the cause of the anemia - beyond mild anemia, further work-up and/or treatment would be indicated

There isn’t enough information in your question. You need to know what type of anemia. What us the hemoglobin level. What else if anything is happening with the baby. Many newborns have mild anemia and no problems.

Yes! There is less RBCs to carry oxygen to the vital organs especially the brain

It is according to what type of anemia and the degree of anemia. If it is severe with the hg 4 or 5 he may need several transfusions. If it is mild his body will usually make its own blood. If he has a genetic condition like sickle cell he may have chronic anemia but this usually doesn't have a low hg until 4-5 months of age.