Offer your forgiveness to him, first. He may feel guilty for not helping you, and forgiveness might ease some of that pain.
Can you do a joint therapy session with a psychiatrist or counselor, so that you both can be honest and supportive toward each other? If there's a trained, neutral third person, that individual can guide the conversation and help you both to understand how to proceed.
Encourage him once more, but don't push him too hard.
Yes, you should definitely care. He's your father, and you seem to be a good son. :D
What kind of abuse did your brother cause you?... And how long ago was it, if it was when you were a kid and you are now in our thirties then it might be time to let bygones be bygones... Did he beat you up?
If you are aware that your father is in depression, and that psychotherapy may help, you may offer your two cents. After all, you are in your late twenties, so feel free to speak to Dad, man to man.