"If he had to be remembered ... it would be for"
I suppose you could say "he would be remembered for" but you can't say "he would be for"
I'm not sure the use of "had to be" is really natural in this case. I think it's more likely you'd hear something like:
"If he's remembered ... it will be for"
It's not quite the same though and depends on what you're trying to say.
Student or Learner