Kill the worm, inspect the dead Hellknights and return to Drezen.
Talk to him and he will send you on a wild goose chase where you need to fight a giant worm.
When Graham the impostor is presented by Anevia, throw him in jail.
Your brother isn't the first to be hardened by war." in the dialogue
Ask Sosiel what Irabeth told him and then pick "The battlefield changes people.
Perhaps the person you once knew is gone.'" dialogue option
When you're back at the camp, talk with Sosiel again and make sure to pick "You don't know what your brother has endured in this war.
Just let it stay on Sosiel's alternate weapon set. Do not sell or drop that shield at any point.
When your camp gets attacked by gargoyles and taken to the Lost Chapel, talk with Sosiel about the fallen Hellknight who was carrying his brother's shield.
It doesn't matter what you say about the cultist that raises the undead
Do Sosiel's quest in Act 2 to start the chain.
These are the steps you will need to take in order to save and recruit Sosiel's brother. The good news is that he can be saved but the bad news is that if you're missing the required dialogue option, you will probably have to wait until the next playthrough in order to avoid him going out in a blaze of glory.
Trever Vaenic is a pretty hard character to save in