Not everyone wants to go online. You can obviously see the benefits (and are likely to be able to manage the disadvantages) but as you have found, your enthusiasm isn’t contagious enough for him to make the move.
There are a number of reasons why people don’t want to go online, such as cost, security issues, physical constraints such as eyesight, concern about their ability to learn the necessary skills etc.. Sometimes too, it’s absolute disinterest and sometimes it’s a timing thing.
Respect his decision, let it go but keep an open mind being prepared to assist should he change his mind in the future. When older people see more of their friends gaining these skills it often gives them the confidence to give it a try too.