My friend when he was 13 slept with his 6 year old brother wearing clothes and penetrated his penis in between his brothers gluten and rubbed his penis on his brother's penis and ejaculate on himself. Is this zina? How to repent from it? What is the punishment? Jazakallah
This disgusting act is not zina, as zina which is punishable if one was caught in the act by enough witnesses or if one admits or reports his act to a qadi is if a penetration has taken place. See also How deep does the penis need to be inside before it's classed as zina and is subject to the hadd punishment?
How to repent: one should stop this and never do this again (for the fear of Allah), feel ashamed for this awful act, and one should ask Allah for forgiveness (tawbah) and do good deeds instead.