Hi Sabrina.
I have 7 kids and I am still trying to learn how to deal with this. My suggestion would be to stick to my guns. Nip it in the butt. I have had a problem with this for years. Sometimes I think that it is my fault the my child lies but then I remember when I was a kid.
Don't take away the after dinner treat but make your son talk to you about it. It will help because most kids don't EVER want to dissapoint their parents.
Just my thoughts.