So, up until he was about 8 years old, he ate both canned and dry food out of a dish? I have seen this kind of behavior in cats, only pulling food out of the bowl to eat.
It could be that something hurts or is irritating when he eats or swallows. A tumor in his throat or the back of his mouth or under his tongue is one possibility. A loose or infected tooth would be another thing I would check for.
Does he ever vomit?
If he were my own cat, I would begin with a good exam of his mouth, which can be difficult. Many cats won't let you look under the tongue, or down the throat, if they are awake so sometimes we have to sedate them to get a really good look.
Since he will eat pieces of dry food from the flood, we know he can swallow so that is good. Dry food has plenty of nutrition and calories, so if he is eating enough food from the floor he can get all the nutrition, calories, and nutrients he needs. However, I would want to get a good look into his mouth and throat, as long as this has been going on.
Let me know if this is helpful and what else I can answer.