You ask what you should do besides talk, but talking has to be the first step. Assuming his drinking was the cause of the problem, is he now willing to try again? If he is, and he remains sober, he may not need Viagra. So that should be your first approach. If he’s willing and can’t, then he should see a doctor. It’s never too late, but you both have to put some effort into it. Twenty years of no sex is a long time and getting your sex life started again is going to take some time and patience. Good luck.