Perhaps the best way is to lay down some ground rules for any future potential partners he may have. Some that I can think of that might help this situation is.
1) He does not pursue romance with anyone without them meeting you first and with him laying out to them that he is in a committed marriage of 25 years, that he loves you dearly, that he WILL NOT, under any circumstances, leave you for another person, and that he will not tolerate any disrespect toward you. They don't have to like you, but they do have to respect you.
2) He fully discloses any conversations or actions from his partners that could be perceived as malicious or threatening as soon as humanly possible to you so that you can take steps to keep yourself and your family safe.
3) He immediately cuts all ties with anyone who behaves in a threatening manner toward you, him, or your children. He will help you to file a restraining order if necessary. He will NEVER see or speak to or contact this person again.
Crazies on the other end aren't the problem. Crazies are pretty easy to dispose of most of the time. Your husband needs to get a grasp on his NRE and make these rules a HARD LIMIT for himself. It is not fair to put you or your children in danger, and he will never ultimately have a good or healthy relationship with someone who is trying to sabotage you or the relationship between you two. The onus is on him to wake up when this kind of thing happens and put his foot down firmly and immediately. I've sent potential suitors packing for far less than that (one just insinuated that I could do better/he could make me happier).
If he is serious about you being a primary and your relationship being his first priority then he needs to walk his talk. He never puts you or your children at risk and he doesn't pursue a toxic relationship with someone who obviously doesn't understand what he is looking for.