Be aware that if you do regain access to his accounts, you absolutely shouldn't carry on using them. You should just withdraw everything in them to your own wallet and hope neither exchange locks the account before your withdrawal is processed. If you try to link a new bank account or other payment method to deposit or withdraw fiat which is not in your uncle's name, then the account will instantly be locked and you will have to go through the recovery methods of completing KYC yourself, providing his death certificate, providing his Will, and so on. Even some simple trading activity after months of inactivity may be enough for them to request additional KYC, which you would be unable to provide.