They are grammatical, but they make no sense! When you ask what's the matter, you don't expect an answer in the present simple, that is, an answer that refers to something that happens habitually, regularly. You need a different tense.
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cbstehIf perhaps the who asked the question was not aware that he is dead, the question will still be valid, such as:True. But almost funny, to my ear! Not an everyday situation, certainly!
Q. What's the matter with him?
A. He's dead; he drowned.
Chris