Oh my goodness. That's a lot of time past.
You know, every relationship there comes a time when you simply just need to wholeheartedly place trust in the person you're with. From what I gather, he's respectful and understanding of the situation and has said its over with the ex and understood why you're rightly pissed off – which you have the right to be too.
Trust him in doing the right thing and forget it. There's better things you can be doing to enrich your relationship to a higher level than to be worrying about some past girlfriend.
“My grandmother always taught me to be happy that my broken toys went to a good home” – the same applies to an ex