A reasonable houserule would be to let him gain utterances, as warlocks gain new invocations.
That was my thought too, only which PrCs could they qualify for, arcane or divine? I would argue either, as Truenaming apparently is neither. For example, you take x levels as an Archivist and x levels as a Truenamer and you qualify for Mystic Theurge, or.... I don't know. Most spellcasting PrCs require spellcasting to entire. I guess your DM would have to be pretty flexible to allow it.
Give 'em both. Its not like it really breaks the class.