The answer I had from a connoisseur is :
Probably before 1910
a gunner
4e Bataillon d'Artillerie de Forteresse (ou 4e Bataillon d'Artillerie à Pied) ,
or 4e Régiment d'Artillerie
or 4e Compagnie d'Ouvriers d'Artillerie
I think it is the boy on the right on the photograph with the father and sons ((Pierre Alexandre?)( the ears seems to match but not those of the boy on the left , nor the father's, and the frontal golfs of the hair too)
gunner could match "brigadier de tir" on Pierre's fiche matricule but there is no trace of a 4 th regiment in the list of the regiments he integrated
Added: I finally think he is Alphonse Guinet , who integrated the 4th regiment of artillery in 1895
.