I'm aware that it comes from a Northern dialect of Middle English as in:
With the full Northern conjugation being:
But in Old English and other West Germanic languages, there wasn't -s or -r in the 3rd-person singular indicative present.
I know that many Northern dialect words come from Old Norse, and a little grammar.
The Norse Conjugation of mæla is:
Ek mæli vér mælum
Þú mælir þér mælið
Hann mælir þeir mæla
And the rune used to write the ending -r in Norse was originally used for the s sound and it remained the rune even when the sound changed, meaning that it was likely originally hann mælis. And there were many Vikings in Northern England.
Is Old Norse a reasonable origin for the third singular -(e)s? Is my little theory sound?