Firstly, make sure you change the 'fastcgi_pass' on the location block I gave you to match yours ("fastcgi_pass 127.0.0.1:9000;") further down as you are using TCPIP and not socket
The 403 will be because of some file/dir permission nginx doesn't like.
From the '/var/www/forum.example.com/' do a 'chown -R www-data:www-data public_html', where www-data = the user you have nginx runnign under (I'm not sure what it is for CentOS). Restart nginx and see if that resolves the 403. If not, then do a 'chmod -R 755 public_html' and restart nginx.