Only God knows the heart and mind of your brother but it sounds as though he may have fallen away from the Catholic Church without recognizing her significance in God’s plan. If such is the case, the fact that your brother left the Church may not, in itself, result in the loss of his salvation.
Quoting from Vatican II’s document Lumen Gentium, the catechism makes it clear that anyone who refuses either to enter into or to remain in the Catholic Church, knowing that the Catholic Church was founded as necessary by God through Christ, could not be saved.
“Basing itself on Scripture and Tradition, the Council teaches that the Church, a pilgrim now on earth, is necessary for salvation: the one Christ is the mediator and the way of salvation; he is present to us in his body which is the Church. He himself explicitly asserted the necessity of faith and Baptism, and thereby affirmed at the same time the necessity of the Church which men enter through Baptism as through a door. Hence 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.” (CCC 846)
