[Maisie Dayz]

You don't say anything about love so I assume it's not a loveless marriage, just a sexless one.
If he's always had difficulty expressing himself, it would be hard for him to communicate the subject of sex and any issues he had and the longer he ignored the problem, the more difficult it became and perhaps embarrassment made him just give up trying.
Help is out there if he can be encouraged to find it.