So my DH’s Spawn was like this, she bragged about lying all the time and being good at it, but her mother was a drug addict so she was coached/groomed from a young age to do whatever Meth Mouth told her to.  As for the secrets that can go along with if they have been groomed/coached by an adult to not share things.

My BD and BS have a pretty good childhood and life, no reason to lie or keep secrets, but they do, it’s just a natural part of being human.  Although it sounds like your SS does it to a whole other level, closer to where my DH’s Spawn is at which could show the child has been abused and has learned to lie and keep secrets to protect themselves.

I actually joined this group for this exact reason...SD20 lying and spying and driving me crazy for 10 years...

Lies for no reason, lies to make herself look better, lies for sport, lies instead of looking up true facts (just makes up stuff).  Literally this has me back in counseling.  And distancing myself.

makes me wonder what he's got on his computer 

I make of it that he has had crappy inept parents.  He does it because he gets a benefit out of it.

Growing up lying was a belt to the ass offense so it was only attempted extremely infrequently.

Have ball-less daddy take a belt to his lying ass and my forecast is that the benefits of lying will immediatley be outweighed by pain and misery.

Re-apply as necessary until he leaves at 18. To lie is a choice and with poor choices should come consequences.

1st, I would check his computer and cell phone.

2nd, I would worried he has been groomed sexually. Keeping secrets, not telling because someone gets in trouble, that is grooming behavior.