All alpha maleism is categorically insecurity. For each of them it's different. Figure out exactly what he's trying to bury under all this tate shit and address it directly. It could be the onset of male loneliness, that shows up around the teenage years - lasts forever though if he's aware he can make changes. It could be he's insecure in his own sexuality - just get him to open up and take everything he said to the grave with you. There could be a person or activity the betas engage in that he can't, or is afraid of being made fun if. Teach him how to stand up to that shit and grow through it.

I'm not suggesting you punish your bro in a way that would enable this behavior further. It's hard to be an alpha type when you're showered in love and support- it just looks stupid. He's probably going through some of the first real life shit, and handling it poorly. As his sister you're kinda on the hook for helping him through this. If you do, you'd probably have a better brother and friend.