On Family Tree do you put Childs last name as their biological father last name or last name of Adopted Father
In this situation either surname would be OK to use, I would most likely use the name the child went by most of their life especially in the case of a young child adopted, he likely legally had the name changed to the adoptive parents. I would use a good reason statement, I would have the other name in the alternate name field and even put something in the life sketch area so it is clear to all who come after me to see and understand.
Here is the guidelines from Familysearch. https://www.familysearch.org/help/helpcenter/article/how-to-enter-names-in-family-tree
They were adopted as an adult but went by mothers married name until they were legally adopted