Sounds like brotherly love to me. If you are not in the school, you may want to look into if he is getting teased or something at school. Or else where. Maybe he cant cant act that way at school, but he can with you. May be why he got scared once he saw that you were going to do something rather than take like before. If youre older, then im sure that set him in his place for a while, if not, then maybe some respect? lol. But im sure it will pass. Just look into him if he is younger. I guess if he is older too. look out for one another.