"할아버님께서 원피스를 입고 계세요."
Translation:Grandfather is wearing a dress.
Imagine you walk into the living room. Grandfather is wearing a dress. There ya go. You wouldn't say Grandfather wears a dress. It is probably an unusual occurence in his case!
The main structure here is V고 있다 which can be translating by 'doing V' or 'Ving'. For example, 입고 있다=to be wearing or 먹고 있다=to be eating.
But here, 있다 is replaced by its honorific form 계시다 (since the speaker is talking about his/her grandmother).
Since "wear" is a state verb, thus "Grandfather wears a dress" should be accepted. "Grandfather is wearing a dress" means "Grandfather is temporarily wearing a dress". Posted on April 1st, 2018.
The ~고 계세요(honorific for 있어요) makes the verb a present participle. So it refers to no time other than the present. This makes "is wearing" the best translation.
I would like to ask you again, is it really common to use a state verb in the progressive tense (state progressive) e.g. "She is having the book", "I am knowing him"?
As per the reference below, "In English the state progressive is ungrammatical. It may be explained in part by was of regarding stative verb meanings as inimical to the notion that some thing is in progress. Formally, Korean has no such constraint" .
 Suk-Jin Chang, "Korean", John Benjamins Publishing Company, 1996, ISBN 9027238049
Quick note, its not correct to state "As Per". Only use Per....
Thanks for your explanation.