The most popular alternative (by far) is OpenOffice
https://www.openoffice.org/
It's widely used by public bodies (such as local authorities) who don't want to pay lots of money to Microsoft. (I use it myself on my netbook. It's excellent).
If your son choose OpenOffice he should note that the default settings are to save files in the program's own formats. Since he will probably want to send files to people who're using Microsoft Office, it makes sense to change the default settings, following these instructions:
http://voices.yahoo.com/how-set-openoffice-writer-save-as-microsoft-11017880.html
Microsoft's biggest rival for office software used to be Lotus. When that firm ceased production of its office suite it placed the code into the public domain, resulting in the development of LibreOffice (as recommended by Hc4361 above). Once again, it makes sense for users to change the default 'Save' settings to Microsoft formats, which I explained a short while ago here:
http://www.theanswerbank.co.uk/Technology/Question1331554.html
A relative newcomer to the scene (but widely praised) is Kingsoft Office Suite Free:
http://www.kingsoftstore.co.uk/kingsoft-office-freeware.html
All of the above are full office suites. (i.e the include spreadsheets, presentation software, etc, as well as word processors). If your son
only wants a word processor, Abiword is very popular:
http://www.abisource.com/