I don't want to upset you anymore than he has already. But you should walk away just because he is a liar! You deserve more than being with someone so dishonest.. If you have to check up on someone and it proves that they are dishonest then there should be no more contact. This is such a heartbreak waiting to happen. Getting more involved will only increase the trips on the emotional roller coaster. He has proven that he can be dishonest and manipulative. You don't want to give him more of an opportunity to do this to you. He is using you to have a fling or as a distraction from his present relationship. Let him involve someone else in that. Just not you!
If this has been helpful press accept