Stress and anxiety can certainly cloud the erection picture but so can alot of physical issues. I'd first see if you can get intimate and just play a game of getting to a certain point where he gets an erection and maintains it - without consummating and having sexual intercourse. I'd do it repeatedly and make sure each of you is getting enjoyment without there necessarily being that predictable "end goal" as a necessary end point. Create romantic evenings, cook together and let the intimacy happen slowly with a build up to a certain point each time.
If the ED persists I would suggest that your boyfriend get checked out since there can be medical causes that need to be ruled out.