problem? He couldn't perform?

My new bf knows that my ex was well-endowed, cuz we were friends long before going out, so he kinda knows the details between me and my ex. I told him a long time ago that my ex was the best i ever had. So when me and my new bf tried to have , he wilted away...He claimed it was just cuz of job stress.. If a guy knows that his gf's ex was "the best," would that put a lot of pressure on him to perform well? What should i do to help him?