It is a new "wrinkle" being he is struggling with cancer. I do understand that you are not happy with him being in contact with his past women/friends or whatever... not an excuse..but he is struggling with cancer - he may want/need support.. I do not know.
You are in a really tough situation. Is it possible to be open with him and let him know that you know it is not in your best interest to be romantically involved with him because his need to hold on to exes.. and it is painful for you. But you do care and you can let him
know "what you are willing to do" as he is going through this difficult time in his life.. discuss an exit plan with him... maybe one of his exes can help