Why are you not blackmailing him by keeping his little secret in return for what you want? Take pictures of everything. Backup in email, google drive, and with girlfriend. Corner him without the rest of the family around when he gets home and lay down your proposal like the new man of the house. He's far too ashamed to let the rest of the family know and will be more than willing to suppress his rage and anger to maintain a stable relationship with the rest of the family. Wife will leave him if she finds out potentially. Could be his worst fear. Ride that angle.

"I snooped in your cabinet. I was curious. I know all of your secrets. I couldn't hide my evidence. I'm worried you'll punish me at a level I am not comfortable with. I've recorded evidence of everything in there. I will tell mom if she finds out. If you kill me, my girlfriend will show her the information. You are not to punish me. You will pay for my college. You will stop being a dick to me (optional, unlikely). We will never speak of this again. You will get a better lock for your safe. Call a locksmith. I don't have money. Here's a number. Mom will never know if you keep your cool. Don't fuck with me on this."

Enjoy results.