>>29
Found out minors are only allowed to write to prisoners if they are in the prisoners family. Also you use a P.O box, which can be provided by church organisations and other communities who encourage such stuff.
Also you can get him to join this site to make it easier, and if he isn't allowed-http://www.writeaprisoner.com/faq.aspx#11
http://www.writeaprisoner.com/first-amendment-letter.aspx
Not sure if that helps at all.