Yes I agree with everyones summations. He isn't honest and I think that just you having to sneek looks at his private mail says volumes about the relationship. If you've been seeing each other for 6yrs then there is something really wrong here.

You should take hold of the batton and run with it (no judgement for hitting him with it either)...There are lots of other men in this world and I would want one who is not going to have another girlfriend in another town or at his mothers place...

This would make my blood boil and it should evoke something like that in you...don't let fear trap you into a relationship that could turn into a marriage that could turn into a marriage with kids that only ends up in divorce because you can't take it any more