At the same time, setting appropriate limits with him and maintaining boundaries of what is reasonable/acceptable behavior when he is interacting with you may upset him, but in the long run will likely help you earn his respect-and as an adult, he should demonstrate some level of respect for you, IMHO. You could tell him something to the effect of, "I do not like it when you lie to me and it hurts me when you do so. If I catch you lying to me in the future, I will immediately end the conversation".