Is your diagnosis fairly recent op?
It sounds like you may still, understandably, be coming to terms with this condition and it’s effects on you and in the future.
I was diagnosed with a similar serious condition, it took me at least a year to accept it, (and by accepting I mean just not breaking down with the fear and unfairness of it all - every moment of every day!) but even then, not totally, I have had many years of good times and bad times coping and not coping with the emotional effects, the fears, the what if’s, and letting go of all your dreams you had up until then about what your life and future could be. As I am sure you will know you have to go through a grieving process.
I agree about speaking honestly to them when you feel that you and your dating partner are getting to be in a more emotional and committed direction, but only so much so that you know that you both aren’t just looking for a casual fling. So that they have a choice, but feelings aren’t too far gone, and so he’s not tricked or manipulated, and then in a position of feeling too attached or guilty to walk away.
There’s no need to say to casual shorter term flings at this moment in time.
I was already in a relationship 12 years by the time I was diagnosed, and even still, over and over again I kept offering my partner a guilt free “out” so he could truly think about wether he wanted to choose to share the burdens and care of all the tough things that come with it. Let me clarify that and say that I mean the situation and the problems are the burden part. Not you, nor anyone else with a serious life limiting illness who didn’t ask for or deserve this. The upside is at this point almost ten years later from diagnosis, is that I am no where near as bad at this point in time than I thought I would be, I am losing more of myself each year, but still hanging on and making the absolute most of my time and relationships.
I hope that you tell your bf now that you have had your eyes open to his feelings and choices in all of this, at 6 months it’s not fair. I do however understand the emotional inner turmoil and perhaps burying your head in the sand to avoid dealing with it. That’s not to say it’s right though. Good luck, and I hope you find someone who will love, accept and take on you in the totality you and of your condition.