YAY, Another Sydney Siter!!! Suddenly I don't feel so alone
Look devilmiss, 6 months is not a lot of time to have a turbulent on and off relationship with anyone. There are clearly some deep trust and resentment issues between the two of you, which have no sign of going away any time soon.
First of all I think you should explain to him that he has absolutely no right to question your motives with other guys while he himself is being so carefree with his ex. If I were you I would even take it a step further next time I was asked a question "Did you sleep with your friend?", My response would be "That's for me to know and for you to find out" with a big smile on my face, just to see a lot of steam coming out of his head (But then again I'm a little braty that way).
Ofcourse, any trust or emotional conflict is best kept in the open. For example, if he has issues with him not being good enough for you, then why? What are his issues? If he resents you for having more than him or is threatened by it, then why does he act or feels that way? The answers to some of these questions are not as complex as we think.
By the way, which area in Sydney are you from? I hope you weren't affected by the riots!