How can I use my sons iPad which was mine originally.

A close Family member or Executor, with the required supporting paperwork, can make application to Apple to be granted access to the Apple accounts of the deceased:

How to request access to a deceased family member's Apple accounts - Apple Support

You should note that if successful, this process will grant access to some information stored in iCloud - but will not, of itself, resolve access to the iPad.

Quite often, close family also have sufficient knowledge - and access to likely recovery paths - that access to the AppleID account of the deceased family member can be recovered. Having done so, it is then possible to release the Activation Lock that prevents reuse of an iPad/iPhone.

The following guidance is written from the perspective of the owner - but a family member will get the idea as they run through the process.

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If your device is Activation Locked, this help page will provide most of the information that you’ll need:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

If you have forgotten your AppleID or associated password, these can be recovered here:

https://iforgot.apple.com/

To recover your credentials, you’ll need access to any one of the following:

• Your primary email address mailbox that corresponds with your AppleID
• Any of the secondary/recovery email addresses that should be configured for your AppleID account
• Any of the trusted telephone numbers (fixed line or Cellular/Mobile) that are associated with your AppleID

Unless you AppleID account has not been fully/correctly configured, or has been seriously neglected, recovery of the AppleID and password should not be difficult.

More information about recovery of your AppleID:

If you forgot your Apple ID - Apple Support

More information about recovery of your AppleID password:

If you forgot your Apple ID password - Apple Support

When you recover access to your credentials, you would be well advised to log-in to your AppleID account from a web browser - and verify/update any email addresses and trusted telephone numbers:

https://appleid.apple.com/

Having recovered access to the AppleID account (and therefore the AppleID username and associated Password), Activation Locks can be removed. There are several routes to this, but for brevity, I’ll refer you to this support page:

Turn off Activation Lock – Apple Support

If the passcodes of any devices are unknown, the data stored on the device itself is now gone. That said, if the device(s) have an associated iCloud backup, you can restore the backup data to the device. To do so, you would need to follow this procedure:

If you’ve forgotten the passcode on your iPad, or your iPad is disabled – Apple Support

You’ll need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). At the end of the process, you’ll have the option to restore data from the backup.

If your intent is to wipe/reset the device(s) for reuse by a new user, you would be best advised to prepare them properly:

What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support

I hope to have covered all the likely scenarios - and similarly hope that this guidance is helpful in fully resolving your current difficulties.

For information about how to request access to a deceased family member's Apple accounts, and through this their equipment, click here --> How to request access to a deceased family member's Apple account - Apple Support