I had some trouble in a previous relationship when I brought up this topic without knowing what I wanted. I thought that discussing this with my partner could be a way for me to figure it out but it ultimately ended up being very shocking and unexpected for him. He was hurt by my having these ideas and considered it a sign that I had stopped caring for him to a certain extent even when I tried to assure him that this was not the case. Revealing these ideas/feelings to my partner, who had not considered them as deeply as I had, came off as threatening. This led me to compromise these interests in order to not lose him when he ended up taking it negatively. If you have an idea of what you need to be happy before you start the conversation, it will make it easier to avoid compromising your needs in order to not lose your s/o.

Before you bring it up to him I would recommend having a clearer idea of what you need and the best way for you to fulfill these needs. It can get very emotionally messy if you open up the conversation without knowing what you want. Is it enough to talk about the feelings you have for this other person with your partner? Do you feel a need to pursue a relationship of some sort with this other person? Do you have a need for more love/companionship or a different kind than what you currently have with your s/o? Ask yourself these questions.

Keep in mind that having feelings for someone else doesn’t necessarily mean that you shouldn’t be in a monogamous relationship. Choosing to be monogamous doesn’t necessarily prevent you from having feelings for other people, but depending on both of your personal needs/goals it may still be the best choice for you at this point in time. I wouldn’t eliminate it as an option because you have developed feelings for someone other than your partner. It is very natural and common to be attracted to multiple people and it is something you ultimately cannot control. Many couples choose to be monogamous while also realizing that their partner might not always be the only person they are attracted to. Weighing the positives and negatives of your relationship structure and how it fits in with your lifestyle and life goals is very important.

Sorry this is so long and not very specific. I hope it helps a little.