Do you have access to the device? Also his Apple ID and password? If you do, you can sign him out and sign yourself in with your ID. When you sign in it will ask you if you wish to retain various types of data. Say yes, keep, for the various kinds you wish to keep. It will then merge this data with your account on the iPad. If you use iCloud it will also add it to your iCloud. I did exactly that 6 years ago when I was given a deceased person's device but asked to keep hold of photos until the family was sure they had them. The family had the device passcode, and back then some of the security measures that you might face now weren't around so it was a bit easier. Even if you do not know his ID, if you do know the passcode and can get into the device you can then use his email account to email the photos to yourself, or use Airdrop to transfer via Bluetooth if you have Apple equipment. How to use AirDrop on your iPhone or iPad - AppleSupport Without his account information you just won't be able to make the iPad "yours" by signing him out of it. For that you would need to do the next set of instuctions.
If you do not have access to his accounts then the standard, but more tedious, way to do this is described in this article: How to request access to a deceased family member's Apple account - Apple Support
