Mental decline after hospital stay?

Hi everyone. I am writing this question in regards of my grandfather. I live with him and help take care of him part time. He has end stage COPD which has been well managed besides these past few months.

After his most recent hospital stay (about 1 week ago, went in on a Friday and discharged Sunday) he is stable in health but he’s not really there, mentally? He hasn’t been talking much, slow to respond and if he responds it’s quiet or not like him at all.

His most serious hospitalization was in the middle of Jan for pneumonia, and he went back after a week of discharge due to a medicine reaction. After all of that, he was fine, back to his spirit, completed physical rehab and was discharged from home nursing care. He was working out and in a really good mood besides loss of appetite.

He went back to the hospital about a month after discharge for allergies we did not know about and caused an exacerbation. He did not go on any new meds besides steroids and an allergy pill, generic brand.

I'm at a loss on this change. Simple questions are met with someone who seems clueless or when we tell him to do something, we have to repeat it. He can still dress himself, walk around, use the bathroom, etc. But the guy inside him is just gone? Sometimes he stares at nothing and it’s scary. This is all in less than a week.

Does anyone have any advice? He did not have hospital delirium in any of his stays and pneumonia is gone. Clear of a UTI as well. Will he come back from this? We are trying to be patient but it’s also really frustrating because sometimes we feel he is just not listening, at all ! Or he just isn’t there. :(

Thanks much in advance.