I don't know but he should be using a flint blade or a rock with some vines tied around it for a handle. They should also make him grunt and point as if he cannot articulate our language as he hops around on his knuckles.
The story could be that Joe found him in a glacier when he was assigned to an arctic outpost and thawed him out. He's Joe's best friend now and helps him survive the harsh terrains where only Matt could feel at home.
And Matt never fails to convey a deep moral message that Joe couldn't grasp earlier on in the episodes but by the end of each of their adventures, Joe finally gets it and is growing as a human being throughout the season thanks to his neanderthal partner.