This sentence is correct but not natural in English.
Aanother translation "what he experienced is very scary"
Yes I believe this should be accepted too.
That which was experienced by him is very scary?
Heavy use of passive sounds odd to the English-speaking ear.
It's not for Indonesian though. The English may sound odd, but it's perfectly natural in Indonesian. Studying Indonesian will require getting used to dealing with passive structures like this.
I think it is odd to hear passive voice in casual settings in English, but it seems normal to me in formal settings like reports or legal situations.
"That wich is experienced by him is very scary".
Given that experienced is past tense I would expect usage of "was".
Normal way in Indonesian native speaker is "Yang dia alami sangat menakutkan"