Hello and thanks for asking this question.  There are many websites to help novice to advanced language learners practice their skills.  Aside form youtube (which believe it or not has some worthy foreign language practice), there are many other websites and at home activities you can do.
 
First, try and learn with him.  Make dinners interesting by offering a meal from a country that uses the foreign language.  Have him read the boxes or cans during food preparation.  Add a foreign language film with subtitles.  Many great foreign language films are easy to follow and are age appropriate for teens.
 
Also, consider going to restaurants that have menus written in his "new" language.  Go to grocery stores, too.  All of these are natural learning experiences which will make the language seem more alive.
 
There are numerous websites, some are listed on college websites to help students practice.  Here are a few that will be helpful.  I am adding them in the text vs. at the bottom.  
 
http://www.Sharedtalk.com   This site is sponsored by the  Rosetta Stone company.  You can get a language "pen pal" via this site.  Recently recommended by my employer ( a local community college) as one of the best sites that offers opportunities for real usage of several highlighted languages.
  
For French learners"  http://www.bonjourdefrance.com
 
For American Sign Language learners:
 
http://www.ASLuniversity.com
 
For Spanish learner's:  http://www.studyspanish.com/
 
 
Of course, with a simple search of the language you will be able to find numerous practice websites that are free.  Try and have him avoid the too easy translators that are available on line.  Sometimes, they are wrong due to the use of the word as an idiom or out of context  They also don't do much for the learning of a language, but might be OK to check if his own translation was accurate.
 
Lastly, consider the many opportunities that schools and colleges offer teens interested in foreign language study, such as learning abroad programs or hosting a foreign student.
 
Good luck and enjoy!  Learning a second language or more is an invaluable tool!
 
Louise Masin Sattler, NCSP
Nationally Certified School Psychologist
Owner of Signing Families
http://www.SigningFamilies.com