*Let* him?!?! Hmmm, let's see. Unless there was something specific in your marriage vows about promising not to *speak* to his ex-wife, I'd would say you would be WAY over-stepping. Jealousy is toxic to a marriage, and the fastest way to get someone to cheat on you is to accuse them of wrong-doing when they are innocent. He may figure, "I'm getting blamed for cheating anyway, I may as well do it."
He is an adult, treat him as such. If you want to ask him what they talk about, or how she's doing, that's fine, but don't let the green-eyed monster overtake you. It is *not* pretty. Trust him absolutely and take him at his word as long as he proves himself faithful. If it was a bitter divorce, then I think it even may be beneficial that they are on speaking terms. It isn't good for your mental health to hang on to unresolved grievances - maybe if they occationally communicate, they can move on, rather than have a heavy heart, holding on to old grudges or regrets.
And yes, I've been through something similar - recently my husband got back in touch with girl he was engaged to about 20 years ago. They broke up under... very sad circumstances and it was a relief to him to know that she's doing much better, and they chatted about their kids and such. Most of his best friends are female, even his "best man" was a woman, and when her husband died, he flew to the funeral ALONE while I stayed hime with the kids. Likewise, I keep in touch with an old boyfriend and male friends. In fact, my husband frequently reminds me that I haven't gotten in touch with so-and-so in a while.
There was just an article on CNN about a woman whose husband was jealous of her male friend, someone who had been her buddy for 20 years. The husband forbade her to speak to him and she said "Look - he's been my friend looooong before I met you." The husband gave her an ultimatum "Either he goes or I go." So she divorced him. BAM! And that's exactly what I would do, too, so... don't "forbid" him - don't snoop, don't read emails. Just ask him how she's doing and be an adult about it.