I would stat him up as a monk/fighter because that looks like a spear being used with dexterity as the attacking ability score and monk is the only way to make that work.
I don't have HBO, but in the books it is identified as a spear. He states it is the only way to counter the Mountain's reach.
"Turned ash eight feet long, the shaft smooth, thick, and heavy. The last two feet of that was steel: a slender leaf shaped spearhead narrowing to a wicked spike. The edges looked sharp enough to shave with."