First of all, why does he feel he needs
Viagra? Is he uninterested in sex?
Viagra doesn’t create sexual interest. Is he worried about erections, or is he not having orgasms? Without knowing his situation, it's difficult for me to comment except in a general way.
Realize that men can have a multitude of sexual concerns with many possible causes. If your partner has lost interest in sex, when did this happen? Is it recent, or has it been building for awhile. Is he tired? Does he work hard?