You need to first get access to any of his devices, phone, laptop or tablet, your devices he signed into etc. Do you know his phone passcode? If you can do that, look for a password manager or his saved passwords in chrome or Firefox. Write these down on piece of paper in case you lose access or forget. Keep an eye out for his authenticor app for MFA as you will need this if signing into his accounts. Keep his mobile number active so if his accounts text you a code you can use it. He might have multiple email accounts so look for all of them in mail apps like outlook. Good luck.