A lot has been said so far but here's my two cents on what you could try.
1. Try to access any of his other passwords. Laptop, PC, paper trail. Try to find physical evidence. See if the same passwords were used on multiple platforms.
2. Is there anyone closer to him than you? Can you ask them if the password was shared? What was their fav thing? Any children? DOB's? Loved places...
Don't forget to try simple passwords like "123456", that was how I was able to access a relative's laptop. You'll be surprised to hear how many people are using these for their OS login since password managers don't work that far.