Anxiety and stress tend to have negative effects sexually for men. At least from my experience. I think it has to do with the body when you are stressed. I think high cortizal and anderline give you the fight or flight response and so the body isn't priming for sex, but ready to run or fight. People are fighting off mountain lions anymore but stress and anxiety trigger the same response.

One option to for him to find relaxing things to do to lower his anxiety.

It may he his medication affecting him. I was on Prozac a couple months ago and I could have sex, but it was very difficult to achieve orgasm. After an hour or so I gave up trying to finish. An our of sex isn't as pleasant as you think it will be.

You could also try Erectile dysfunction medication. They help increase blood flow and aid in mainaining an erection. They also aid and not cause an erectio so sex can still happen naturally. I have never taken them so I don't know how well they work.

You could also try physical things like penis pumps and cock rings. They work by trapping blood in the penis.

Best of luck to you. I think honest, open communication will help you in this matter. Sex can be difficult to talk about, but sometimes it helps to know what your partner wants. In long term releationahips we get into the trap of thinking our partner knows what we want and we never actually say it. Sometimes we have talk about it, even if we ourselves think it is obvious, our partners may not know.