Do you remember when this started with him?
I have a friend who is a teacher. When her son was the same age as yours, he would go to school and tell his teacher (also a friend of his mother) that his mom made him stay in a closet for hours every day after school. The boy's teacher confronted her friend, the mother, who was horrified over her son's lying.
When I taught preschool (4-5 year olds), several often shared with me unbelievable stories about their home life. Andy's dad's car could jump over other cars to get ahead in traffic. Sara's dad, a policeman, she told me, was regularly shot on duty (and maybe off duty). My own sons went through a similar phase of lying and thinly-veiled truths as they tried to astound and amaze their father and me.
I'm guessing this is a phase for your son, as it was for all the kids I've mentioned. As much as we hate it, lying and fibbing are part of a child's development as he individuates from his parents. Here's an
article about this that includes suggestions for dealing with it.
If it's more serious and longer-lasting than this, then you will have explore other alternatives.