Welcome to Using English, maving. What a great thread title!
The standard idiom is to fall on hard times so we'd say 'He's fallen on hard times'.
We don't use 'to be on/in hard times'.
Student or Learner
If someone is not doing well money-wise, do I write:
"He is in hard times."
"He is on hard times."
You could say "He's having a hard time financially".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.