Why can't my boyfriend keep an ?

First off my boyfriend is about to be 39 years old tomorrow and I'm 27. We've been together for 7 years. My boyfriend used to be a stallion in bed, he could go for hours, very well endowed, and very sensual. However, for the past 4 months he has been unable to keep an . I admit I do have a high drive. However, he states that he is getting older and has a lot of things on his mind. I asked him is he not attracted to me anymore he said that was ridiculous. I'm not going to lie it's starting to bother me that he is no longer able to keep an . He told me that this is the first time this ever happened to him. My boyfriend stated when he was in his 20's he would an just looking at a woman. He suggested maybe I should find someone else to satisfy me, but I don't want to since I love him so much. How can I help him get through this.