He's cheating on his gf with you. Call it what it is. He is seeing you on the sly, lying to his gf because he know what he is doing is not acceptable. He's using you and if the tables were turned he's probably cheat on you with someone else. Cut him off completely. Don't just not return messages; tell him you can no longer communicate with him in any way. You really are just setting yourself up for a great deal of heartache. He's not calling you his girlfriend. That is important to remember; it is very telling of his motivations.