My baby brother doesn't believe in Santa anymore, what should I do?

My baby brother who is 9 stopped believing in Santa because a friend told him and had to ruin it right before Christmas. It kind of saddens me, even though I am 13 and stopped believing 2 years ago I still have a sad feeling about it. He constantly brings it up and I always reject it even though I know the truth. I don't want him to ruin it for other kids his age because it is just unkind and ruins it for everyone. I believed in Santa until 11 which is pretty old and I am grateful I held on for that long, I am just sad it had to end for him this soon. Why am I so upset by this? How can I fix this?