Need HElp!Extention for L-2 visa or change to F-1 visa

[orlov012]

I have a question about my son .I recieved L-2 visa extention for him just until 01/20/2007(my husband has l-1 extention until 06/01/2008), he will be 21 on this date.Now he applyed for college submittion to Los Angeles Pierce College for the Winter to Fall 2007 Session, and recieved student identification number .
Do I need to file extention for L-2 visa,or I have to change to F-1 visa.
Could he apply for social security number on L-2 visa?