I have a brother who is adopting a child and he wants to change his surname to ours. Please can you tell me if it's Haraam to do this as he my brother is a religious guy and he asked and they told him it's ok as long as he don't put in the birth certificate that he is the father, but I want to make sure as other people told me it's Haraam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions. 

If what is meant by changing the name of this child is to make him be traced back to you then this is forbidden, as Allaah has forbidden this. Allaah says (which means): {…nor has He made your adopted sons your real sons.}[33:4]. Islamic legislation forbade us to trace ourselves back to a false family lineage. The Prophet said: "If a man is named after other than his father while he knows it, he becomes a non- Muslim. Besides, whoever traces himself back to a people with whom he has no family lineage, let him find his place in Hellfire." [Al-Bukhari] However, if what is meant by changing his name is something else, then you have to clarify this to us so that we may give the appropriate ruling about it. Anyway, it is safer to trace back this child to his real family lineage if it is known to you, otherwise he can be given a general name without specifically tracing him back to a given family lineage, for example naming him so and so Ibn Abdullaah, or so  and so Ibn Abdurrahman and so forth.

Finally, we remind you that adoption is forbidden; on this regard, please refer to Fataawa: 82371 and 84422. 

Allaah knows best.