Yes, it is fine, and it is a very natural way to say it. I think the grammar is simply that "hard" is an adjective modifying "worker." Checking ahdictionary.com, the list it explicitly as the third meaning of "hard" as an adjective--and they actually use "hard worker" as their example of use! 3. Proceeding or performing with force, vigor, or persistence; assiduous: 'a hard worker.' Here are a few examples of use from written literature: "You seemed to be a hard worker..."--F. Scott Fitzgerald, "This Side of Paradise" "You speak of difficulties on Natural Selection: there are indeed plenty; if ever you have spare time (which is not likely, as I am sure you must be a hard worker) I should be very glad to hear difficulties from one who has observed so much as you have."--Charles Darwin, in a letter. "He knew Spiegel to be a conscientious hard worker."--C. S. Forester

As Fidel says, "he is indeed a very hard working person" is, of course, correct. But "he is a very hard worker" is also, very definitely, correct. Maybe it shouldn't be correct, and I hear what Fidel is saying, but the fact is that it is correct. :) I'll try and explain why it's correct, and someone else can correct me if I'm wrong. In "he is a worker", "he" is the subject and "is a worker" is the predicate. That predicate consists of "is" (the copula, or coupler), and the complementary noun "a worker". The copula connects the subject with the noun. "He" isn't a worker in the same way that "he" is a human. Rather "he" is a worker because "worker" is the value of his occupation property. "Hard" is an adjective, and an adjective can be used as a noun modifier; here the noun being modified is "worker". So the sentence is of the form <subject> <copula> <noun modifier> <complementary noun>.

No, it isn't. Because "hard worker" is kind of the subject itself, the noun. For example: "A hard worker is a person who delivers its work in a very efficient way..." But if you're trying to say it as an adjective, you cannot say that he is a hard worker, you have to say: "...so, that said, I could asure that he is indeed a very hard working person". //// Hope I could help, cheers from Mexico!

Ok, updated. :)

Thanks Fidel and Steve. Yes,Steve. Look forward to your informatiom. Thanks again.