I recently read an article about lying in a local parenting magazine and what i took from it was that it is important to talk to your child about the "cost" of lying and then encourage them to find other ways to meet their goal.
It also said that kids sometimes lie because they don't want to get in trouble and if the parent shifts that piece then sometimes the child is more willing to let go of the lying.
Is your son able to write out words? Maybe when there is a question about his truthfulness you could say something like, we are going to take a break from this discussion,I don't want to get upset with you, but I do want you to be truthful because (insert your resoning) so I would like you to think about what we've talked about and if you need to change what you have said you can write a note on a piece of paper and bring it to you or put it in a special place in the house where you will check it later and then you can talk about it.
My almost 6.5 year old loves to write up menus, lists, etc and even though I can't understand most of his spelling it's fun.
And maybe having a plan B for when the one on one isn't working would be helpful?
Good Luck
