Why would this guy be acting like this?

Some people are just pathological liers. It is a mental illness from which some can be healed. Some people get to a point where they're not even aware that they're lying or they forget to whom they lied what! You were not mean at all. You certainly don't have to waste your time with this person. If you care about him so much, you can talk to him in private and tell him what's going on and suggest tht he may want to seek professional help? Don't do it though unless you really are concerned for him.