Yes, it's Belgian. It's called a Velodog or pocket pistol, from the days when dogs used to run after folks on bicycles, pestering them until they fell off. This one dates from after 1892.
It's a sad little gun to bring back from a war, but if that's how he got it, then that's how he got it.
Sadly, it is a cartridge-firing piece and therefore not the kind of gun we discuss on this forum. Up to 1865, I bleeve, and a muzzle-loader, and you're good to go.
This one is near the turn of the 20th C and is therefore taboo here.
Got any muzzleloaders?