About 40 years ago as a childless Mekubal and a fatherless young man grew ever so close, as a father to a son and vice versa, they mutually decided, reflecting their relationship, to change the “family name” of the young man [to e.g. Binyominson, where Binyomin is the name of the “father”], calling the young man as his son.
Could it be appropriate for the ‘son’, missing a biological father’s name with which to be called up to the Torah [and had therefore been called up as “ben Avrohom”], to be called to the relationship Torah as “ben Binyomin”, after the ‘father’?
Furthermore, with the ‘father’ having “passed” with no biological children or brothers etc, can the ‘son’ stand-in as an inheritor?
Note: Relevant observations that come to mind include “Serach bas Osher”, the teaching regarding one who raises an orphan ‘within his household’ [vis a vis inheritance], the general approach of “public knowledge” and/from or a father claiming one to be his son, serving to identify a son to a father [as opposed to two eidim of birth, or what have you..], and of-course, a declaration from Yaakov Ovinu affecting full inheritance to both of Yosef’s sons along with his own.
Thank You Very Much.
P.S. This takes into consideration surviving testimony to the above account.
Based only on what you wrote, I am assuming that the child was brought up by the childless couple. Regarding if the adopted son inherits is controversial. Here are some of the sources for you to research.
The sefer “Al ben imatzta lach” 18-3 brings Shvus Yackov 1-169, Maharam Shick O:CH 58, Mishna Halachos 10- 207, that the adopted son does not inherit, while Maharashdam 341, brought in Nachlas Tzvi that if it would be clear that this is what the niftar would have wanted then he does inherit, also see Shut Chasam Sofer E:H 77, and Shailos Ya’avetz 1-165.
Another factor mentioned by the above mentioned sefer, is that if the “child” wasn’t brought up in his home, then everyone agrees that he doesn’t inherit him.
Regarding calling him after his adoptive father, see the above sefer chap. 7, that this is also controversial, however there are numerous opinions that it is permitted, especially in this case that the natural parents name is not available. See sources and comments
See Tzitz Eliezer 4-22-1 that if the couple doesn’t have children he may be called by the name of the adoptive parent, also see Nishmas Avrohom 5 pg 136, that if the real fathers name isn’t known then the child may be called with the adoptive parents name. Also see Shut Chasam Sofer ibid., Igros Moshe Y:D 1-161, Lev Aryeh 55.