Unfortunately Master Password relies very much on your ability to remember your personal secret. This trade-off is necessary to put the owner in full and exclusive control over their passwords. If the owner is not able to remember their master password, they have effectively lost the proof that they are who they claim to be.
What you can do at this point is either:
1. Try to work with your father to remember the master password (which I'm sure you've tried).
2. Reset your master password. Doing so will change all of your dad's site passwords, so you will need to update all of them, such as by using their "forgot my password" links and setting a new password to each website.
You can reset your Master Password user's password by long-pressing on the avatar icon when trying to log in. If you remember the old master password later on, you can also reset to go back to the old master password and you'll get all your old site passwords back.
An emergency strategy to protect yourself against forgetting the master password might be to write it down and put it in a safe place such as a personal vault. Just keep in mind that this is your master key, treat it with care.
When choosing a new master password, try using a memorable absurd sentence. Log in and log out a bunch of times, forcing yourself to type it in and practice it a lot. Write it down and destroy the paper for practice. Whatever works well for you. Wait a few weeks and repeat the practice once more, for a more time-enduring memory.