Believe him, it is not you, it is him. Stress and anxiety can definitely cause this. And now that it is an issue, it is just making it worse. Make sure he is well rested and relaxed before sex. And remember he does not have control over it. It is an automatic reaction to sexual stimulation. So it is not as if he is rejecting you. I can almost guarantee you he is just as frustrated as you are.
He should cut out bad habits, like smoking, drinking heavy or drugs (if he does any of it). Does he exercise and watch his weight? If not, you might want to work out with him to encourage him to look better after his health.
Also stop trying so hard for a while. It is perfectly fine to just cuddle, have oral sex or finger sex for a bit. Get some massage oil and massage each other. Intimacy is about ore than sex, and if you are frustrated, you always have your fingers (or even a vibrator) to hep you out.
Sex after a night's rest might also help with stress.
If nothing helps, he should see his doctor. It might be an indication of more serious problems. Just trying ED drugs by himself without seeing a doctor may just hide other problems.
Best of luck!