Forgetting Bluetooth device on iPad but not iphone

You appear to have two principal (but related) issues...

1) Outside of Educational and Enterprise establishments, iPad and iPhone are principally intended to be a single-user device - and are not intended to shared between users. If your Son is using an iPad that is signed-in to iCloud with your AppleID, you can expect to encounter difficulties. Apple's Continuity Services are designed to switch tasks and some accessory devices seamlessly between a user's devices as they are being used.

2) Related to #1 above, after initial pairing with a device, Apple's AirPods and Beats headphones will automatically pair/switch to the User's currently in-use device via Continuity. If two different people are using devices that belong to a single individual, you will encounter issues. Insofar as the devices are concerned, one user is using multiple devices.

Your remedy is straightforward. Your Son should have his own iPad - fully independent of your own Apple devices - that will be associated with his own AppleID.

forgetting bluetooth devices one 1 device should have no bearing on what other devices the said bluetooth device have been paired with

How to Delete a Bluetooth Device on an iPad - Solve Your Tech