Apologize for the long thread ahead

I have a FWB for a year that I am really close to. He is 9 years older than me. I feel very attached to him, we've been seeing each other weekly, often more and do many things together. We like to spend several days together. I confide in him a lot. Obviously this is not just a fling, and I've certainly had some apprehensions, but I feel like it's about time I ask if he wants to be my boyfriend. We are so compatible in many areas. The thought of potentially letting him go in favor of dating other people breaks my heart.

However, I'm interested in having an open relationship. I feel I still want to explore sex with other people, while romantically he is my rock. We have great sexual chemistry, that's not an issue. I fear missing out on sexual experiences though, and this doubt especially has been holding me back from committing to the relationship for a long time. I also have a feeling an open relationship would feel like a more authentic way to have a relationship for me.

I want to tell him so much how I really feel about him, but we always have to downplay everything and act discreet to keep things platonic.

I have a strong feeling that we would enjoy exploring sex with others together as a couple. I have a feeling an open relationship could be good for the both of us, and we could manage it as there has been no jealousy between us so far, despite me dating others openly and sharing about some of my conquests. I haven't actually had any romantic feelings for anyone else during this time, but I have been fooling around. We haven't been exclusive in the slightest, and I'm fairly certain he has focused on me for this whole year. He was quick to get off dating apps when we established our situationship and finally labeled it a FWB thing. At that time we were both unsure about entering any kind of committed relationship and had some apprehensions about the age gap when things were so fresh.

I have thought a lot about how this request would come across as offensive while simultaneously asking for a committed partner, and frankly I'm not sure how to ask respectfully, but I also want to be up front with him. I feel a solid way to go about it would be to be up front, but still agree to having a closed relationship for some months, and really talking it through while we settle as a couple.

I'm having a bit of a spaz right now and my feelings are all over the place. First of all I'm worried of ruining a good thing, he could just as easily just break it off. Secondly I have no experience with ENM. I made a similar thread on an unrelated forum, and there was an army of people claiming fwb and open relationship is the same thing so they thought it pointless. I think it's not the same at all. I want to commit emotionally, romantically and to a future together.

I've never heard of going from FWB to a committed open relationship, only closed relationship to open relationship. I'm curious about others experiences, and could definitely use some thoughts and advice.

Ps: If relevant I'm INFP he is ISFP, I am anxious and he is secure, we both have touch and quality time as primary love languages. We are both men.