A fever for  2 days  is most likely due to some kind of infection.  If the baby has other symptoms like a runny nose or diarrhea, than that could be a clue to where the infection is coming from.  If the baby is not having other symptoms, you would want to get a check up with your doctor to look for other reasons for infection like ear infection, throat infection, pneumonia, or urinary tract infection.  In the meantime,  be sure to give the baby plenty to drink so he stays hydrated.