he needs to be evaluated
Welcome to healthcare Magic!Steroids
have many adverse effects when taken for long. Prednisolone
is also a steroid. You have not provided the reason for which steroid is needed for him.
Elderly people are very susceptible to hyponatremia
that can cause altered consciousness level.
He may be having some other age related disease like Parkinson's or Alzheimer disease which commonly lead to low slurred voice.
Prednisolone is very unexpected drug to cause loss of voice.
I would like him to be reviewed by a local physician and he will be able to decide whether he can be switched to steroid tapering or sparing regimens.
I hope the advise would be informative and useful for you.