Thought he didn't like the spartans and wanted to go a different route? Anyway i guess he'll keep us on our toes.
He could be lying so in order to not spoil it for the non comic book fans who don't know anything about Star-Lord. Also, in the first episode of the GOTG animated series it is revealed that Peter's father is a Spartan. And yes, I know that the GOTG animated series isn't a continuation of the movie, but it is taking a lot of cues from the movie. So based on how Marvel has been bending over butt backwards to make the comics and cartoons resemble/reflect the live action movies, it's a safe (but not guaranteed) bet that this might be the case in the movie (with a few changes).