Student or Learner
He went to Turkey on holiday.
He is on holiday.
The above is correct.
Maybe he will go to Greece after holidaying in Turkey.
This is not a single holiday if he goes to Greece.
He is on holidays.
Is the above correct if he goes to Greece?
I'm not clear about what you are asking. I will say that while "holidays" is very common, but I really don't consider it correct. "Holiday" refers to your vacation; it is pretty awkward to turn that into a plural unless you are reminiscing about all the wonderful "holidays" you've had in the past. When a person says they are going on "holidays", I cringe a little.
So, I like all of your examples except for the final; "He is on holidays". Very common usage, but really not correct.