Gender: boy
The best match I found when searching for Sz. Senior was Slzama Senior born 1926 and he lived at Brunnen 18 flat 3 and TTEL 13. He would have been the correct age for the Gymnasium and High School.
Arja/Aria Lajb Senior born 16/01/1905 or 16/11/1905, Raca Senior born 1901, and Eliasz Senior I believe are Szlama's mother, father and brother.
I found Szlama Senior on JewishGen "Mauthausen inmate list of Jews born in Łódź" it says he was born 25 Jan 1926 in Lodz, Poland, worked as a musiker, his prisoner number was 134173 and he arrived in camp in 1945 and he died 6 April 1945 in Solvay, Ebensee.