Caligirl,
From my experience, I think what makes this harder for you is the ommision of what was going on involved money. As a newly single mom who went from a very cushy lifestyle to trying to raise two boys on my own, on just my salary, (because he would not make child support or daycare payments), oh I understand how tight money can be.
From my personal experience, my five year old, who normally never lies, just told his Dad on the phone the other night that the "Practice Lockdown" the school had was because there was gunman outside their Kindergarden class window with a shot gun and tons of police had to come. Whoa! Talk about a whopper! My STBXAH immediately had me on the phone to explain what the heck happened. Took a while to calm him down but when I explained that he would have seen it on some news if it had happened, he got it. (Don't worry we had a long talk about telling the truth)
He was told a little about the Sandy Hook incident from an older child at after school care. Nice....Anyway, I think at around this age they still are not all together sure about right and wrong and truth and make believe. Use this as a learning tool about that just because he did not tell you does not, make it not a lie. (Bad Grammer!) And that when money is involved that it can affect things like food for dinner, new shoes, being able to pay for TV bills, etc. (Stuff they will relate to.)
Unless your son saw something directly related to an incident like this, I think it was most just your son making a bad decision.
Good luck Mom, it is not always easy,
4MyBoys