"Ever after" is a poetic way of saying "from then on". It would be perfectly fine to say "nobody saw him from then on", but I'm hesitant to say your example is correct because I've never seen it used with something that didn't happen.
It is usually used in a positive sense, that is for things that did happen, for example:
- They lived happily ever after.
- That was the name by which he was ever after known.
- She remembered him fondly ever after.
It isn't normally used to describe things that never will happen, as in your sentence. For example, it does not sound idiomatic to say "he wasn't known by that name ever after" - we would just say "he wasn't known by that name anymore".
"Happily ever after" is something of an exception, because of its perpetual use in fairytales it is has become a phrase in its own right, and people do say "they didn't live happily ever after".
In place of your suggestion I would perhaps say:
- Nobody saw him ever again
- He was never seen again
- He was never heard of again