Well the problem that I see is that he is being SECRETIVE about communicating with his ex. That's the difference. If it's so harmless why be secretive? Is it because he does not want you to do the same thing? Just as he forbid you to communicate with anyone else? That's just wrong.
If a couple wants to have friends of the opposite sex than it should be their friends TOGETHER, not secretively. Of course there are people with differing opinions.
My husband and I really don't singularly keep in touch with any old flames because we know it would be a problem for the other. But basically we are quiet and not very sociable people. I like it that way.
I don't know what to suggest, I just know I would not like his behavior because yes he's acting like a hypocrite. And why such tenderness for his ex when he should be more focused on you because you're his fiance'?