Yesterday onward my grandfather's mouth kept open and remains same till now. What should I do further?

We can't tell much from the information given. Your gf's mouth may be open to make it easier to breathe. It may also be open because he is nearing death. Having the mouth open is more relaxed and requires less energy. Tell us a bit more and people may be able to help more with their answers.

He could have suffered a stroke. It's imperative that you get medical attention for him if he is not speaking or eating or if he cannot speak the words he's trying to say.

Hello, you give very few info about his general condition, so not easy to reply. I hve worked as home assistant for elderly people, and only have seen this phenomenon when people were sleeping. The most probable cause is that he has nasal problems so that he is obliged to keep his mouth open simply to breath. But then most probably he also has problems drinking and eating ?? Have you consulted your general practician ? Has he been checked by a specialist (mostly combined mouth / throat and nose). The problem also can be situated in his mouth or throat, and be letting in fresh air by breathing through his mouth, may be the pain is softened. Very sorry but apart from these possibilities, I can not see anything else. Of course I understand that you are worried, but I think the best thing to do is to contact your general practician first, and in case of need go to the hospital to a specialist. Hope all comes well and with a big hug from Belgium / Europe

My mother is a mouth breather, she always has her mouth open unless she's eating. In this case it's no big deal, but we don't know your grandfather's condition.

Call his Doctor, maybe it's time to call hospice

More information needed to answer this question.

Is he having difficulty breathing? Is he conscious? If not, I would call 911 so that they can check his vital signs. You can refuse transport to the hospital if that's what he indicated he wanted, but then you might want to call a local hospice organization to see if they can help you with some in-home comfort care.