My dear, the very fact that you and your husband have been together for 28 years tells me most of what I need to know, here. No two people can be together that long, unless there is a very real, true bond between them...
It sounds to me as though your husband is mourning the loss of his youth. The fact that he is exhibiting interest not in some woman he's just met, but, rather, in women he knew as a youngster, is quite telling. Indeed, he may think of these women often, yet, I imagine as he's thinking of them, he sees himself, as he was, when he was young, and his life held so much promise for the future. In a way, he may look at these women as a way to re-connect with that long lost youth, by sharing with them, old remembrances of the way they all were, long ago. He's going through the old, 'What if?' , of his life...looking back, and wondering if possibly, he might be in some better place, than he finds himself now, had his choices been different. What is transpiring has nothing to do with you, it's 'all about him'.
Unfortunately, this is something he will probably go through, no mater what you do, or say. The truth is, the only human being we ever completely, truly know, the only human being over whose actions we have control, is our 'self'.
www.intouchwithjeannine.com I believe you might find the following article of interest, my dear:
http://friends-lovers.com/articles/midlife.html