To do this you will have to restore the entire backup, not just the single app. This will replace the data and settings on the iPad with the data and settings in the backup, losing more recent data. If you only want to restore the app and aren't concerned about the app data (such as his game progress on the app) you can just redownload the app for free, avoiding restoring the entire backup (as explained here: http://support.apple.com/kb/HT2519). However,if you want to restore the app data, you will have to restore the backup.
Restoring to an iCloud backup has to be done in the initial setup process, which requires first erasing the iPad. If you have any data on the iPad now that is not in the backup that you want to save you'll need to do that first. Then go to Settings>General>Reset and tap Erase All Content and Settings. You will then go through the setup screens as you did when the iPad was new. When given the option, choose Restore from iCloud Backup (see image below). Be sure you are connected to wifi and your charger as this will take some time to finish.