It's common practice for visual media to associate one name with one character -- unless the plot specifically calls for something otherwise, in which, eventually, that new or those other new names will become "universal" as well. This is done so that the audience can keep track of the characters, but it does become extremely jarring when a character would say or refer to a name that would [plot-wise] not mean anything to them.

SOLVED (for me, at least) Summon him via Companions & Contacts (N) menu. He'll be under Followers -> Main Characters, and he'll be the only summon-able NPC on that list.