Yes, he's a courier, and it's heavily implied by Joshua Graham that Caesar either has or has intentions to hire him (most likely as an assassin, rather than a courier).
too quote J.Graham " I've heard one of them (frumentari) travels the wasteland as a courier. Most of Ceasers agents meet a fitting end in NCR territory, but maybe this one survived" this is said after meeting daniel and giving him the suplies. when J.Graham first meets the player he mistakes you for 1 of ceasers spies also disguised as a courier.
Ah, thanks for adding that quote, and correcting what I said. I misinterpreted the story a bit there, I thought the situation was reversed. Thanks! :) -Poster 1