Yes, Tolkien has used "human" to refer to men

Tolkien is fairly consistent in his published works, but you asked if Tolkien has ever referred to Men as Humans, and he has, especially in some of his more out-of-universe essays, where the word "human" is often used in direct contrast to "elvish", or as synonymous with "Men" or "mortal".

Here are a bunch of examples.

Then the Eldar forbade them to land on Eresse, for they were of human kindred and mortal; and albeit the Powers had granted them long life, they could not release them from the weariness of the world that comes upon all men ere the end
_{Sauron Defeated - "Earlier versions of Edwin Lowdham's Old English text"}

It is in fact simply part of the portrayal of the imaginary world of the Silmarillion, and an example of the kind of thing that enquiring minds on either side, the Elvish or the Human, must have said to one another after they became acquainted.

...This, he would say, was a known fact concerning Elvish nature, and could therefore be deduced for human nature from the close kinship of Elves and Men...

...They therefore deduced that this was 'natural' to Men (se. was by the design of Eru), and supposed that the brevity of human life was due to this character of the human fëa: that it was not designed to stay long in Arda...

...the change in the condition of Men from their original design was due to a primeval disaster, about which human lore is unclear...

...but both became 'mortal' and died later according to the normal human span...

...But there are few records of any contacts of Elvish and Human thought in such latter days...

_{Morgoth's Ring - Tolkien's "Commentary" to the Athrabeth Finrod ah Andreth}

...it is based first upon Elvish records and lore about the Valar and their own dealings with them; and these have reached us (fragmentarily) only through relics of Numenórean (human) traditions, derived from the Eldar, in the earlier part
_{Morgoth's Ring - "Notes on motives in the Silmarillion"}

Thus to a Man Elves appear to speak rapidly but clearly (unless they retard their speech for Men’s sake), to move quickly and featly (unless they are in urgency, or much moved, when the movement of their hands, for example, may become too swift for human eyes to follow closely), and only their thought, perception, and reasoning seem normally beyond human speed.
_{The Nature of Middle-earth - "Time-scales"}

The “ages” of Quendi are usually given in terms equated with human life; but not all lengths of years had the same ageing effect upon the Quendi.
_{The Nature of Middle-earth - "Time-scales"}

Only in one known case, Beren, did the Valar – by special permission of Eru – restore a human body to life and suffer its fëa to return.
_{The Nature of Middle-earth - "Elvish Life Cycles"}

Any element of an Elvish strain in human ancestry was very dominant and lasting
_{The Nature of Middle-earth - "Beards"}

Many, though given in apparently Quenya or Sindarin forms, are not found in the Elvish or Human tongues of Middle-earth.
_{The Nature of Middle-earth - "Of the Land and Beasts of Númenor"}