He has "a ways" to go? [duplicate]

In English we often say, for example, "he still has a ways to go before he's done." Is this grammatically correct?

AHD4 has the following entry for ways:

ways n. Informal (used with a sing. verb) Variant of way. See Usage Note at way.

The Usage Note states, in part:

In American English ways is often used as an equivalent of way in phrases such as a long ways to go. The usage is acceptable but is usually considered informal.

As an idiom, the phrase seems well-established in the United States. As Peter Shor said, Ngram shows evidence of this. The Corpus of Contemporary American English gives me 138 hits for "a ways to go" and 193 for "a way to go". Many of these hits are from published print sources. (In the British National Corpus I get no hits for "a ways to go".)

As a native English speaker, I think "a ways to go" sounds fine.

EDIT: Also, see page 949 of the Merriam-Webster's Dictionary of English Usage.

"Ways" is not the plural of "way" but a separate word with its own meaning. "Ways" means "a certain distance, not great, perhaps, but significant"; it is often modified as "a little ways," which makes it mean "a very moderate distance." The distance may be physical or figurative. "Ways" is singular, always seen as "a ways." On this understanding, your usage is grammatically correct.