I know my son is a liar
What gives you the idea that he's a liar
When I told him to clean his room, he started putting off my question saying that he was doing something important.
I'm sorry you feel that your son is lying to you. You can't be really sure about what is going on unless you talk to him. Maybe he was doing something important at the time.
I know he's lying, I don't care anymore.
I'm sorry you are feeling that way. It's morally wrong to deceive another person. It is just best if you try to be open with him and figure things out.