Now I hasten to point out that what I'm about to say may not apply to your grandfather but: it may not be anything "mechanical" that is affecting his ability to eat or to see, but simply another stage in the dementia. Obviously teeth problems could be the reason, but many people get to a point where they simply forget how to chew, and when the eyes themselves can be functionally perfect but the messages don't get processed properly. This may not be the case here, but it is possible.
If there are issues with his swallowing he should be seen by a speech and language therapists: strangely these are the people who deal with swallowing issues.
If there are issues with his swallowing he should be seen by a speech and language therapists: strangely these are the people who deal with swallowing issues.