Have you seen this Ted Lecture? Your husband is perfectly described in it. An astoundingly large number of very young men have been watching porn videos over the Internet and masturbating to an extent and degree that they now have problems with ED. Viagra doesn't work because it's not a physiological problem. TEDxGlasgow - Gary Wilson - The Great Porn Experiment - YouTube
Know this: porn addiction is a variation of Internet point / click addiction. You need someone specifically trained in these areas to help him overcome this.
Most men like porn and nearly all men masturbate, so it's hard to convince someone that they have a serious problem with porn that is deeply interfering with their life. ED in a young man is NOT normal and is nearly always attributed to the issues you describe.