It sounds like you have automatic downloads turned on. If you turn it off on your devices, his app downloads will not appear on your devices.
Settings>iTunes and App Stores>Automatic Downloads>Off for Apps.
You can create a new Apple ID for the shared iPad, but if you want to be able to share content with each other, that can present problems when you start downloading past purchases using another ID. You will associate the device with the new iD and lock yourself out of the other ID for 90 days and this will occur every time that you switch ID's and download past purchases. Read more here..
iTunes Store: Associating a device or computer to your Apple ID