Why do you think Jon Arryn took Hugh as his squire? Hugh didn’t seem to be from a noble family.

He’s not from a high-ranking family with a recognizable name, but he could easily be the son of a household knight or other retainer from the Eyrie. Certainly Ser Hugh was poor and without prospects other than Jon Arryn and becoming a tourney knight, but that’s not necessarily a good metric of whether or not someone is highborn or lowborn.