After reading this I feel sadness for your brother. Not because he has left his faith, but because he is being forced into an impossible situtation by his own parents. Imagine if our Heavenly Father handled all of us the way they are him. Would we not be resentful and angry?
Someone said earlier that it's like death for a child to leave the church. No it is not. As a person who one has left and came back and two as a person whose son has left when I left, it is not the end. I came back. I have no idea if my son will come back or not. But that does not change the fact that he has been taught the truth, that he is still a wonderful good person (despite not being Mormon), and has a great deal to contribute to our Heavenly Fathers plan. Does it mean that I am not sad he left, well I am more sad that it was my influence that drove him out. I suspect your parents are mostly feeling guilty and are more angry at themselves than they are at him, but are taking it out on him. I learned early on that I could not do that with my own son, or I would loose him in my life...and what good would that do either of us?
Just because he is choosing to live the life he is living now, doesn't mean in time he won't change and come back. Lecturing and berating him now will do no good. Even the wise can not see all ends.
And...and.....the love of God and the gift of the Atonement still apply to your brother....maybe that's what your parents need to remember. After all he is a son of God and has been long long before he was their son. And God loves him and wants him to return much more so than any mortal human can understand.