It’s possible that your brother isn’t culpable. Whether a Catholic who leaves the Church and dies outside of it can be saved, the answer is that such a person can be saved if they were innocently ignorant of the truth of the Catholic faith. This would be the case for a person who was raised Catholic but never truly catechized and who had seen no evidence of sufficient evidence for the Catholic faith. Such a person could be innocently ignorant.
However, if a person at any time makes an act of Catholic faith (that is, places his faith in the teaching authority of the Church so that he will believe whatever the Church teaches because it speaks for God) then God will give him the grace to persevere in the Catholic faith. Therefore, if he withdraws his assent from the Catholic faith, he is sinning against the grace God has given him and thus is not innocently ignorant.
Thus, the only way for a Catholic to leave the Church and still be innocently ignorant is for him to never have made an act of Catholic faith in the first place, which is a real possibility in this age of inadequate catechesis, in which some people are never taught that the solemn teachings of the Church are infallible. Thus the Church teaches that “…they could not be saved who, knowing that the Catholic Church was founded as necessary by God through Christ, would refuse either to enter it or to remain in it [LG 14:cf.].
Your brother’s rejection of the faith is a difficult cross to bear. Continue to pray for your brother’s repentance and return to the sacraments. Don’t be discouraged, but, instead be joyful in hope, patience in affliction, faithful in prayer (Romans 12:12). “No prayer is ever lost”—St. John Vianney.
A prayer for your brother: “I pray also that the eyes of your heart may be enlightened in order that you may know the hope to which he has called you, the riches of his glorious inheritance in the saints…” Ephesians 1:18
*Search and Rescue: How to Bring Your Family and Friends Into, or Back Into, the Catholic Church * by Pat Madrid: shop.catholic.com/cgi-local/SoftCart.exe/online-store/scstore/p-B0414.html?E+scstore