He has a girlfriend but gets jealous if I hang out with other guys?

[Clair]

I met him when I started uni in September and we hit it off right away, exchanged numbers and ever since have been talking everyday and meeting up when we are both in uni.

He told me he didn't have a girlfriend when we first met but he did say there was a girl in uni in Scotland who told her mum and dad he was her boyfriend but he isn't. He always hides his phone when he gets a text from her and went to Scotland to see her and wasn't texting me as much. Also she commented on his status on Facebook and her profile picture is of him and her.

Anyway he is still telling me he doesn't have a girlfriend but I don't know whether to believe him or not. He is always telling me he likes me and now beautiful I am but hasn't done anything about it but it even though I have given him plenty of opportunities. But if I hang out with other guys he starts an argument with me and then doesn't talk to me for hours later. This has happened with a guy who is my lab partner at uni and a guy I have been friends with since I was a kid.

If he has a girlfriend why is he getting jealous and arguing with me when I spend time with other guys?