yep. Harming animals for fun puts him on the fast list to doing the same to people.
He needs to get away fast, and He needs to get to get to the other side of the country, get a second floor apartment, and a gun and take lessons on how to responsibly use it, and regularly go to the firing range so he knows how to safely load, handle and shoot it. and if he gets comfortable with it, maybe even a concealed carry permit if its available where ever he ends up.
Because if his brother does come for him, the chances are extreme that only one person is leaving the situation alive, and I think we would all prefer Op to be the survivor.
And, one last thing, is to stay off social media.
I would argue to not give any info to ANYONE. Not family, not friends, not your boss, nothing. Abusive families can be manipulative and experts at putting pressure on others to get what they want.
I disagree. Certified letters wont do much of anything.. I would suggest stopping on the way out of town by a local police office and tell them that you are escaping from an abusive and dangerous home life, that theres a strong chance they might try to report you missing, and you are here to put on the record that you are leaving for your safety, you are not missing, because they are likely to try to use the police to track you down. . . Assuming you can trust the local PD, and they have no connections to your family that would be useful for tracking you down.
This has to be a major priority once he gets whereever he's settling. Take classes, and go to the range regularly to get used to loading, handling, firing it, and the recoil and noise.
I would add one last thing.
Do nothing on your phone. You don't know if tracking software or logging software is installed on it. Throw away your phone, and buy a new cheap PoS phone with a whole new number, from a completely different carrier than what you currently have. and do that while you are still in town, so they can't use it to find out where you are, or get a general direction on where you're heading.
The same could be said for your computer, too, but thats more easily reformated. you could even get an immutable OS setup so that all it takes is a reboot to restore the system and undo any tampering.