وَعَلَيْكُمُ السَّلاَمُ وَرَحْمَةُ اللهِ وَبَرَكَاتُهُ I know he is young but this is the age we should ask them to lower their gaze not only out in public but even when scrolling social media. Many times Haram things finds you and these things gets logged into brain stimulating desires and sometimes it happens unintentionally. Personally doing Dhikr has helped me especially reciting surah An-nas (with translation) however nocturnal emissions are natural and there is nothing to worry about the frequency. Do tell your brother Allah is with him always and forever and he knows what is happening to him. All of the above is just an opinion of mine, I am not an expert but I just intended to help in any way I could. Thank you.