Thank you for sharing your thoughts on this thorny issue, which, one way or another, has been discussed quite a few times in the past on here. There is, of course, no right or wrong answer to the question of whether any particular individual would share this sort of news with someone else and this is particularly pertinent when the ‘significant-other' is a potential partner.
From what you have written in your last sentence, it would seem that you are already doubtful about letting anyone else into your life at present. Preparing yourself to have an ileostomy might be what you need to make up your mind about whether you tell this man the reason behind your contemplation about ending the relationship.
If you don't want to (or can't) confront him with how you feel:
Had you thought about writing your feelings down (as you have already done in your post) and then you don't have to cogitate on what you might have to say face to face. All you then need to do is decide whether to present him with your written account.
Of course, if it was me in this position, I would probably present the whole dilemma in rhyming verse, as this is my preferred way to work through difficult emotional and social problems. What I have also found is that if I have not already made a definite decision on a particular dilemma, I would write down the many questions that are running through my head and then make some attempts to answer those questions from as many perspectives as I could. This literary method gives me and the potential reader (which is sometimes me-at a later date) a much more comprehensive idea of the background to my thinking. Sometimes I come up with ‘conclusions', but more often than not I end up with more questions, which remain either unanswered or unsatisfactory. It is often surprising how useful it is to have written things down like this so that you don't have to keep rethinking but can re-read your own thoughts on the subject and perhaps add or subtract elements so as to have it try to make more sense in the longer-term.
It may be that your ‘friend' would appreciate being informed of the situation, so that he can either try to help you during this difficult time, or he might decide that he does not have the ‘stomach' or the expertise for such an endeavour. Whatever ‘he' decides might be what you are looking for in terms of not having to make those sort of decisions for yourself.
As I have said earlier, there are rarely any right or wrong answers, so my comments are just my own ramblings which may or may not be helpful. When this sort of thing arises, I often contemplate the remark made by George Bernard Shaw: “Ninety percent of all the things I ever worried about - never happened!”