Can or should I ask my significant other to stop talking and to not associate with his ex? If so, how?

Perhaps help him see how that makes you feel (or state why else this is affecting you so much) instead of instructing him not to do it. If the idea to not talk to his ex comes from himself, he might feel better about that action.

It depends, you should know better than me since I dont know him. Exs sometimes mark us through a part of our lives, and if the relationship ended up well sometimes the good bond stays even if the love is no longer, if your significant other is still good friends with their ex and you dont feel like they share other kind of feelings with one another then it may be harmful for them to give up a close friend. When it comes to this things, you have to aknowledge your feelings and to talk with your SO about them, let him know the situation and work it out with you, share your insecurities and trust you and him can get to a situation where you are both comfortable with each other

This really depends. I don’t feel like I’m close enough or informed enough to pass judgement on this situation correctly. However, I do personally think that if this ex is particularly harmful, abusive, problematic, toxic, or otherwise bad for you/your significant other/your relationship, then it is reasonable to want them to stop talking. If you feel this strongly about something involving your relationship in general, you should always speak to your partner about it. A lack of communication in a relationship is a death sentence for it. I’m sure if you explain your (reasonable and fair) reasoning, they will understand and want to do their best to preserve their relationship and respect your wishes.

When your partner keeps in touch with his ex, it can stir up anxiety, resentment or concerns for you. That's a pretty normal reaction. It turns out a recent study shows that "people are more likely to communicate with ex's they still have feelings for." Especially if the contact is frequent - several times a week. But what do you do? A good start is to come clean with your own feelings without accusing - "I need to share something I'm feeling. When you spend a lot of time with your ex, I start to wonder if there's something missing for you in our relationship. It makes me uneasy and anxious. Can we figure this out together?" Good luck!

Trusting your significant other is important. Talk to them about the way it makes you feel and you can decide together if it would benefit your relationship.

Just sit down and talk with your partner in a calm, polite way and explain why you want them to do so.

If you believe that asking them to stop talking to his ex, then ask him nicely but always remember that e might be friends with his ex still, don’t get mad if he says no.

Of course you can if the situation makes you uncomfortable you have a right to ask. Just start by mentioning the subject and then slowly build up into the question of why they find the need to associate with their ex and if they would mind stopping because of how it makes you feel.

In my personal experience if you are feeling unsettled or uneasy about him communicating with a past lover. Try addressing your concern with him and your unease, collaborative communication often gives the best results.

Be open with him, the key thing most people don't do in relationships is communicate. Communicating is a key factor in relationships, and it's a great way to be open about each others concerns or what ever it is that is going on. If this is really bothering you, express it with him.