Explanation:

Correct Answer is (3).Replace 'as' with 'since'.
In this case, the intention is to convey 'from the time' he came, so the correct thing to say is 'since he came here'.
'Since' is used in two ways -
1) To express time 
2) To express 'cause' or 'reason'.
e.g. It is a month since I saw her. (Time)
Since she is my aunt, I obey her. (Reason)
The correct sentence should be: He has been studying with me since he came here.
'As long as/ So long as' denote time during which an action or event takes place.