If you go into a conversation assuming that you already know the possible outcomes, that conversation is doomed to failure.
A conversation where one participant has already decided the possible outcomes is either an order or an argument. Neither one is conducive to a healthy relationship, especially since you don't actually like the outcomes you see.
Instead, identify the conflicts that you perceive to be causing stress in your relationship, and the goals you wish to achieve while resolving these conflicts.
Be prepared to be flexible on your goals. You need to make space for your partner's goals as well, and some things may not be totally achievable.
In this case, it looks like your conflict is that you and your partner are no longer sexually compatible (or at least not compatible enough). Your goals are to resolve the stress this is causing and avoiding a breakup. (I'm putting words in your mouth here, so feel free to adjust if you feel these don't actually match your goals).
I didn't include "avoid forcing him to have sex on your terms" as a goal because I considered that part of "resolve the sexual incompatibility", but it might be worth stating outright. This conversation isn't about you having a problem, it's about the two of you having a problem together, and finding a solution that works for both of you. But being extra clear that the solution you are looking for takes into account your partner's happiness should help establish that this conversation isn't just about you, and will encourage your partner to participate.
Once you've organized the problem in your mind, then do the same thing in the conversation - lay out the problem, establish your goals, and then brainstorm what you might do for a solution. At each of these steps, make sure that you and your partner are in agreement before moving onto the next step.
It's okay if in your conversation you can't find a way to achieve your goals as long as you decide on some way to move the problem forward. For example, agreeing to couples counseling or trying something sexually that neither of you has done before. Neither of those solves your problem by itself, but trying them might lead the way to a solution in the future.
Starting the conversation with a reluctant partner
Your partner has been avoiding having this conversation, likely because they see the same possible outcomes as you, and they like them even less than you do.
Don't try to initiate the conversation all at once. Establish your desire for the conversation, and then let your partner pick the time and place for it. Giving them time to prepare and letting them choose a location and time where they feel comfortable and secure will help the conversation go well. Again, during this initial planning step emphasize that this conversation will be a joint effort for a joint problem.
Once a time and date has been picked, set it in stone (barring emergencies, of course). When picking a date and time, try and pick one that is unlikely to have a conflict arise. Similarly, pick a location that is unlikely to have unexpected disruptions or distractions. Turn off your phones (or at least put them on silent) for the duration of the conversation.
You've made an assumption that is wrong
I don't know what it is. You don't know what it is. But your understanding of this problem is flawed in some way. Be open to figuring out what that is.
What we have here is a failure to communicate
Sometimes people have true conflicts in what they want or desire. But just as often they simply misunderstand that they both want the same or very similar things, but are expressing what they want differently. Pay attention and try to identify where you and your partner are miscommunicating rather than conflicting. A miscommunication is neither person's fault, but both can try to fix the problem.