The thing about public schools is that your child gets compared against all other children. Congratulations here! He is doing great! The trouble with home schooling is that you have your own expectations and he has his - and really neither of you gets to see a overall picture of what's going on.
So yes, it is kind of a phase. I would call it (in my opinion only) more of a communication phase.
I learned long ago as a teacher, parent,middle school vice-principal, elementary school principal to completely ignore lying. I quit asking who did it. I either caught the child and gave the appropriate punishment or let it go. Knowing that I would be extra alert and catch them next time. Smart kids lie. It's self preservation. Heck big business, oil companies, clergy, elected officials do it all the time.
The point is that the lying is not the worse thing. His action that led to him lying is the bad thing. When you start punishing him for lying, you confuse the whole issue. Yes, morality wise - it would be wonderful if he didn't lie. When you figure out how to teach morality let me know - cause a lot of people have sure missed out on that one. So - if you can - pretty much ignore the lying. Deal with what is he has done. Keep it simple. Be consistent, fair, and immediate in your punishment for his action. Once he quits doing it (and it won't happen overnight), the lying about that action will stop.
And by the way, you are not the only ones that feel violated about lying. Lots of people on this forum feel the same way. My son is 25. If he lied to me, I would be very hurt. But if he did- there was something else going on and that root cause is what has to be dealt with.
Hope this helps - and makes sense - its a bit hard to put into words.
I'm not a doctor or anything, but I am very educated on the subject of ADHD/ADD (I am a proud one myself!).
It seems to me that your child has a big problem with impulse. Combine that with the fear of consequences, and you're really pushing it. One factor of ADD/ADHD that's overlooked (my opinion), is impulsiveness, which can be a HUGE behavior factor. Whether it be verbal impulses, or decision impulses (succumbing to peer pressure), it definitely comes into play more often than not.
I had the impulse problem growing up (still do and I'm almost 21 - not verbally anymore though), and one thing that I clearly remember my doctor saying to my parents is that negative consequences do no good, as instilling fear in a child with ADHD can do a lot more damage than good.
What goes through the mind (in regards to his lying) is a simple rapid-fire response that is barely comprehended. He doesn't give himself time to think like a person without ADD/ADHD would. For example, if he's doing an instant message chat online, he has a lot of time to think and isn't as impulsive and can better communicate his thoughts.
With that being said, positive reinforcement is the way to go. I know it sounds foolish and I'm not a parent, but try having him communicate written responses if you feel he's lying and I have a feeling you might get something out of him that's better respected.