Was never sure. Was it Kwentrith's son? If so, by whom?
From the summary on wikipedia of season 4 episode 14:
Ragnar states he and Kwenthrith never had sex and Magnus is expelled.
The boy Aethelwulf sent away was Magnus, the son of Kwenthrith because he has no value if he is not the son of Ragnar. However it is not said in season 4 who the father is or if Ragnar lied.
Here is a picture of the boy just before he left :