i google the title and author name and found it in wikipedia:
"Games" (1953). A boy becomes the characters in his make-believe games. Originally in Galaxy Science Fiction (March, 1953). Available online
according to wikipedia, the magazine galaxy science fiction existed until 1980.
you wrote that you contacted a french publisher?
and they couldn't help you?
there is something wring, as a publisher has to contact the company, who inherited the rights of the magaizine, and/or the family.
no company can publish something without this - you yourself are trying to find out.
do you have a local artist interests group or administration?
maybe they can help?
(in austria and germany, we have something like this and can even pay a fee to them, and they organize everything. also when informations are needed: that would be easier, instead of you trying to find out, who has the rights.)