> John is eating little less salt than usual.

This one isn't correct; it would always be:

> John is eating **a** little less salt than usual.

The description of the quantity should be able to stand on its own as a noun. So if someone asks:

> How much salt is John eating, compared to his usual amount?

You would answer:

> A little less.

And not:

> Little less.

Similar phrases would be:

> A great deal more.

> A lot less.

However, "little" seems to be somewhat of a special case, in that we don't need to say anything like "a little **amount**", "a little" suffices of its own to describe the quantity.