I am going to disagree with my esteemed colleague on this because you need only assign a separate email address on his iPad as the contact address for FaceTime. It does not have to be an Apple ID. I did this for months until all of my family members got our own iPhones and now we use the iPhone numbers as our contact at address.
This video explains how to do it.
I tried that before, and it would not work. Assigning a different Address in the "You Can be Reached By Facetime At" section assigned the address to all the devices under the Apple ID there. So when attempting to call that address, the call would fail. Removing the address on one device, would remove it from all the devices.
I was only able to call my iPad from my iPhone when I changed the Apple ID for Facetime to a different one.
But I will not discredit your experience Demo. There's no harm in trying it out anyway.
Have him use an email address like gmail.com. If both devices are using the same address, it's like calling your own phone.
Unable to make or receive FaceTime calls after April 16, 2014
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
iOS: FaceTime is 'Unable to verify email because it is in use'
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage
How to Set Up & Use iMessage on iPhone, iPad, & iPod touch with iOS
Set Up Alert Sounds
Extra FaceTime IDs
Troubleshooting iMessage Issues: Some Useful Tips You Should Try
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Fix Can’t Sign Into FaceTime or iMessage iOS 7
FaceTime, Game Center, Messages: Troubleshooting sign in issues
Unable to use FaceTime and iMessage with my apple ID
iOS 7 allows you to block phone numbers or e-mail addresses from contacting you via the Phone, FaceTime, or Messages
How to Block Someone on FaceTime
My Facetime Doesn't Ring
How to watch FaceTime calls on the big screen with Apple TV
Send an iMessage as a Text Message Instead with a Quick Tap & Hold
To send messages to non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8
How to Text on the iPad
How to Send SMS from iPad
How to Receive SMS Messages on an iPad
Apps for Texting http://appadvice.com/appguides/show/apps-for-texting
You can check the status of the FaceTime/iMessage servers at this link.