New boyfriend cannot finish

I recently started dating a new guy. He is 20 years older a than me, but in great health and shape, as his job is in the fitness industry. We are very comfort ale with each other and everything is great, except the fact that he can never finish. He loses his erection after 15 minutes. He claims this never has happened to him before and he is dumbfounded as to why this is occurring. He keeps reassuring me that it's not me but obviously as a female the insecurities start creeping up. Any suggestions on what I can do to help this issue? Thanks guys!