Are there any dinim of aveilus for a child that was adopted when his adoptive parents pass away? Are there any poskim that spoke about this?
The Nishmas Avraham (Chelek Daled Siman 42) discusses this. An adopted child is obligated to honor his adoptive parents in order to express gratitude for the kindness they did for him, but does not have the the actual mitzvah of honoring parents. He should say kadish for them but not at the expense of the biological children. He is not obligated to tear kriyah, or sit shiva, and even if he wishes to do so he is still obligated to learn Torah whenever there is no one visiting.