A difficult situation. I can see why this is hard for you. I would not be comfortable in this particular set up either.
He, and she, have decided to stay in this uncomfortable relationship and deal with their incompatability by keeping the status quo.
Have you told him that you are fearful he will not protect your time with him? Has he taken away that time before? If he has, then that's a problem with him that you need to address directly with him. You mention you've told him your fears and concerns which is a good idea. How did he respond?
Does he do anything that makes you insecure? Does he ignore you? Not keep his agreements, cancels dates? Things like that? Or is it the situation and your own thoughts about their situation that makes you feel insecure? From your writing, it seems to me to be the latter.
If this is accurate, beyond asking him to respect the time you have set aside for each other, your feeling diminished or dispensable are not his to solve. They are yours. It is certainly not wrong to ask for reassurance occasionally. But he cannot fix a basic feeling of insecurity or worthlessness. Is this a momentary twinge of insecurity (happens to everyone) or an indication of deeper concerns? You're only one who can know this.