You're using the same apple ID and probably both using his phone number in Messges as the contact address. In other words, you are sharing the same phone number and that won't work. Add another email address to your iPad and use that as your contact address.
Watch this video for more information.
You should use your Apple ID for iMessage.
Settings > Messages > Send & Receive > Apple ID: > sign off
Sign in again with your own Apple ID
Create an Apple ID with a valid e-mail address below:
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
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8