Easier said than done, but I would relax about this and let it sort itself out. Although he is PG, he is also five. Doesn't have all the tools for sorting his feelings and preferences out yet in a manner adults buy into. I'm not even sure I would call what he's doing lying, which has moral overtones, of course.
FWIW, we have a five year old as well. Not tested, but very bright (his brother tested as PG, we're guessing he's up there, too). I'd say he is going through something akin to a regression period. Lots of small and large tantrums, hard to direct, rebellious, almost distracted acting, for lack of a clearer term. Otherwise, he's a delight. ;-)
We are currently chalking it up to the impending transition to K.
