I think there is some cause for concern because of the intimacy of their conversations and the amount of emails etc.
I doubt there is any physical involvement as your husband sounds like he loves you and would not intentionally put your relationship at risk by physically cheating.
Could I suggest when you have your conversation with him you remind him of the essential values that made up your loving and enduring relationship ie, Love, Respect, Honesty, Trust, Caring, Humility, Communication.
He may not even consider that he is showing a degree of disrespect to you by having such a close relationship with this young woman, also his lack of communication with you regarding the intimacy of his conversations is something for concern.
He sounds like he is a little smitten with her either, or he feels like a father figure to her.
Anyway all good wishes for the conversation with him.
Please keep us in the loop,
🙏