Well; his size means he's likely neither assassin nor caster, Jackson's already the Archer so he can't be that, a Saber would expected to be carrying a way fancier sword, and he's not a girl which most Sabers seem to be. He's wearing armor Jackson would likely recognize as something more attuned to old-fashioned cavalry as opposed to direct melee, ruling out berserker.

Plus a class isn't as secret as the name; the class would be obvious the moment they do battle.