Pardon me if I've missed something in this thread, I didn't read the whole thing. But I did a quick search for child development and lying.
Here's an excerpt:
Six year olds lie for many reasons; most of them are developmental, but others may be stress related. A six year old’s "lies" are not exactly like those of an adult, because a child’s understanding of right, wrong and morality are different than an adults. However, since six year olds are developing their sense of justice and morality, it is important that we be sensitive and thoughtful when we respond to their experiments with truth and mistruth.
Six year olds are still trying to figure out the power of language. You can make things happen with language: you can draw a picture for someone of an event they didn’t experience, hurt someone’s feelings or share a thought, an idea, a feeling. You can create a whole reality with language. Once children discover this, they’re driven to explore it, and much of their lying comes from this kind of wishful thinking: "I wish this were true, so I’ll pretend that it is, and then maybe it will come true."
Children are also creative. They create elaborate stories that are similar to dreams; they have parts in them that are true or based on real events, but they also have terrific flights of imagination. Younger children are often unsure which parts of their stories are real and which are fantasy. As they get older, they are clearer about what is true and what is false, but other questions arise.
"Can I use language to change someone’s idea of an event?" "Could I actually change a past event by describing it as different than it was?" "If I can convince Mom that I didn’t take the cookies out of the cupboard, would it still be wrong?"
In light of your husband's addiction, I would agree other posters that seeing a therapist is a good idea. I would avoid anything that might shame your son. And approaching therapy with an attitude of "Is there something wrong here?" rather than "There is something wrong here with this lying" may avoid burdening your child further.
He might have a tendency already to feel like something is wrong with him because dad can't fill his needs for love, attention and consistency due to the addiction. Kids can't imagine there is something wrong with a parent, they can only see themselves as having done something wrong. Lot to deal with for a little one.