How should I approach this?

Hello,

I have been seeing this guy for about 4-5 months. Things are great...sex is good however he can't cum. He gets aroused and sometimes even has precum but he can't seem to finish. I have brought it up a few times, he has told me it's psychological and it has nothing to do with me and that he enjoys me physically and he is not worried. I thought the block was that he was afraid to get me pregnant so I went on the pill as an added protection but still nothing. Is it me? Should I be worried? Is this somewhat common and I should just let him be and bring it up in a few months? I know not everyone needs to cum to enjoy sex and intimacy but I also can't help get in my head about it. The last time brought it up he told me it's not an issue and that I should look for something to worry about.